On a side note, what is the need for five devi's when there already are the Tri Devi?
The five forms that the Devi as Para Brahman assumes are called Panch Prakriti viz are Durga, Laxmi, Saraswati, Savitri(Gayatri), Radha.
As you asked about, what was the role of these if already Tridevis were present?
If you see Durga, Laxmi, Saraswati are Tridevis and are already part of Panch Prakriti. Thus, I think you missed that. Now, moving to the role of Panch Prakriti, I'm answering the question in three parts, first where a short hint is given to roles for a quick go through, in second part are the citations from Devi Bhagwatam explaining roles of each Prakriti in detail, and third is the meaning to be understood at depth.
The role that of Durga in Panch Prakriti is to protect those in distress, who are surrounded by enemies.
The role of Laxmi is to provide wealth of all kinds.
Saraswati, plays the role to give knowledge.
Savitri, is the deity of Sandyavandas, and rules over chandas of vedas, thus gving Brahmgyan.
Radha is the giver of Rasa (highest estacy)in life and Bhakti for Krishn.
The citations used are from Shrimad Devi Mahabhagwat Purana, 9th Skanda, 1st Chapter- On the description of Prakṛti, Shlok 4-66.
Ps- Not all shloks from 4-66 are cited here, only those highlighting the role as related to question are mentioned. Thus, in case you want to read the whole subject, then please refer to Shrimad Devi Bhagwatam Puran with aforementioned Skanda and chapter.
Nārāyana said :-- “O Child! Who is there in this world that can describe fully the characteristics of Prakṛti! However I will describe to you that much which I heard from my own father, Dharma. Hear. The prefix “Pra” in the word Prakṛti means exalted, superior, excellent; and the affix “Kṛti” denotes creation. So the Goddess, the Devī Who is the most excellent in the work of creation is known as the Devī Prakṛti. By Her command came out five Forms of Her, either for the purpose of creation or for bestowing Favour and Grace to the Bhaktas (devotees).
Durgā the Mother of Gaṇeśa, comes, as the first, the most auspicious, loved by Śiva. She is Nārāyaṇī, Viṣṇu Māyā, and of the nature of Pūrṇa Brahmā (the Supreme Brahmā). This Bhagavatī Durgā Devī, (when She gets pleased) destroys all the sorrows, pains and troubles of the Bhaktas that have taken Her refuge, and gives them Dharma, everlasting name and fame, all auspicious things and bliss and all the happiness, and, the Final Liberation! She is the Greatest Refuge of these Bhaktas that come to Her wholly for protection and are in great distress, whom She saves from all their dangers and calamities. She has infinite qualities.
The second Śakti of the Paramātman is named Padmā (Lakṣmī). She is of the nature of Śuddha Sattva (Higher than Sattva Guṇa) and is Kṛṣṇa’s Presiding Deity of all wealth and prosperity. This Devī is residing in all the grains and vegetables and so She is the Source of Life of all the beings. O Nārada! All the lovely beauty that you see in all the living beings and all the things, it is She; She is the glory and fame of those that have done good and pious works and it is She that is the prowess of the powerful kings. She is the trade of merchants, the mercy of the saints, engaged in doing good to others and the seed of dissensions in those sinful and vicious persons as approved of in the Vedas.
This third Śakti is named Sarasvatī. She is all the learning of this endless Universe and She resides as medhā (intelligence) in the hearts of all the human beings; She is the power in composing poetry; She is the memory and She is the great wit, light, splendour and inventive genius. She gives the power to understand the real meaning of the various difficult Siddhānta works; She explains and makes us understand the difficult passages and She is the remover of all doubts and difficulties. She acts when we write books, when we argue and judge, when we sing songs of music; She is the time or measure in music; She holds balance and union in vocal and instrumental music. She is the Goddess of speech; She is the Presiding Deity in the knowledge of various subjects; in argumentations and disputations. In fact all the beings earn their livelihood by taking recourse to Her. She is the bestower of the fruits of the ascetism of the ascetics; She is the Siddhi and Vidyā of all; She grants always success to all. Were She not here, the whole host of Brāhmins would always remain speechless like the dead cluster of persons.
She is the mother of the four colours (castes), the origin of the (six) Vedāmgas (the limbs of the Vedas and all the Chandas, the Seed of all the mantrams of Sandhyā vandanam and the Root, the Seed of the Tantras; She Herself is versed in all the subjects. Herself an ascetic, She is the Tapas of the Brāhmins; She is the Tejas (Fire) and the caste of the Brāhmin caste and embodies in Herself all sorts of Samskāras (tendencies; inclinations); She is the Japam. Pure, known by the names of Sāvitrī and Gāyatrī. She resides always in the Brahmā Loka (the Sphere of Brahmā) and is such as all the sacred places of pilgrimages want Her touch for their purification. She is purely Śuddha Sattva, of the nature of the Highest Bliss; She is eternal and superior to all. She is of the nature of Para Brahmā and is the bestower of Mokṣa. She is the Fiery Śakti and the Presiding Deity of the Brahmā Teja (the fiery spirit of Brahmā, and the Brāhmaṇas). The whole world is purified by the touch of Whose Feet, this Sāvitrī Devī is the Fourth Śakti.
O Child Nārada! Now I will describe to you about the Fifth Śakti, the Devī Rādhikā. Hear. She is higher than the Highest; the Essence of all, infinitely superior, the First of all, Eternal, of the nature of the Highest Bliss. From Her has sprung the Rāsa maṇḍalam and She is the Grace and the Ornament of the Rāsa maṇḍalam She is the Presiding Deity of the five Prāṇas; She is the Lady of the Rāsa Līlā, the Foremost of the Jovial, humourous (witty) persons and dwells always in Rāsa. Her nature is the Highest Bliss, the Highest Contentment, and Excessive Joy; She transcends the three Sattva, Rajo and Tamo Guṇas and is Nirākāra (without any particular form); She assumes forms only to show Her favour to Her Bhaktas. She is the Giver of Bhakti (devotion) towards Kṛṣṇa, services towards Kṛṣṇa; and She bestows all wealth and prosperity.
Now, talking in literal sense, Durga not only protects one from outer enemies in Samsara but also liberate the soul from the six vicious inner enemies like kama, krodha, lobha, etc. And winning over outer and inner enemies is the foremost thing one wants in spiritual life. Because, you can't expect to sit in any sadhana when someone is bothering you at your doorstep nor can you contemplate upon your isht when you are battling your own evil side.
Laxmi devi provides wealth, now even food, clothes, etc are a part of wealth. Even to do yajna wealth is required in the form of Yajna Samhita, Dakshinas, etc. But, she not only provide the material wealth, but also spiritual wealth like giving good companions who are lined towards Dharma, etc. Everyone needs Laxmi, even virakt sadhakas. Gyan is laxmi, being Nirogi is laxmi, having the determination (dhairya) too is Laxmi.
Saraswati gives knowledge. But knowledge in this world is of infinite types. Like learning to make explosives is also knowledge. But, the role of Saraswati is that she leads her Sadhaka towards Brahmagyan. She gives them the ambrosia of Shastras which are the sole true knowledge. Music, Kala, Dance, Poetry, everything outflows from her. It's because of her that today with us we have so many Natya, Gadya, Kavyakhandas. She not only bless with kalas, but make sure her sadhaka uses it for Dharma, no doubt why sanatana flourished so much ! Now just reading some black words is what anyone can do. She make those pure thoughts of Shastras influence person in real life. That's what true knowledge is.
Savitri is the teja of mantras, tantra, beejas and chandas, she is deity of Sandyavandana. But, in real sense its only by her that one aligns their breath and experience highest reality. Sje is called the Tej of Brahmā, where any attribute of Brahmā is Brahm itself. Thus, she herself is the Brahmroopini, who gives the real Brahmgyan to her upasakas. Thus she has more to do than rather being simply a Sandhya deity.
Radha is well known for her Rāsa. People tend to misunderstand what rasa is.
What actually is Rāsa?
The Rāsa is a very old concept and is found in the Upanisad in the sense of pure consciousness or the highest reality.
Abhinavagupt presented a scholarity and philosophical exposition of the rāsa experience in the light of the Dhavni theory of Anandavardhana. There is undercurrent of spiritually in all his writings. According to him the rāsa experience though never identical with but similar to Brahmasvada because at the time of rasa realization the aesthete or the connoisseur goes beyond the limitations of the ego-centric experience.
When a person crosses the limits of egocentric experiences, he attain ParmBrahm the ultimate reality. Thus you see, Radha gives the ultimate experience of ecstasy in life.
Thus when these Panch Prakrutis come together, the individual moves rapidly towards liberation.