My answer will be a little different from the traditional answers.
Devi Maa is the Shakti that helps Lord Vishnu to perform the duties.
She Upholds and Supports the 3 Wide Stepping Vishnu along with others.
3-6. I am the five elements as also what is different from them. I am the entire world. I am the Veda as well as what is different from it. I am the unborn; I am the born. Below and above and around am I. I move with Rudras and Vasus, with Adityas and Visvedevas. Mitra and Varuna, Indra and Agni, I support, and the two Asvins. I uphold Soma, Tvastir, Pusan, Bhaga, The wide-stepping Vishnu, Brahma, and Prajapati. To the zealous sacrificer offering oblation And pressing the Soma-juice do I grant wealth; I am the state, the Bringer of Wealth; Above it all, place I its protector. (Devi Upanishad).
She helped Lord Vishnu slaying Madhu Kaitabha.
Mahabharata [Book 6 (Bhishma Parva)-chapter 23] states, “I bow to thee that art fond of battle! O Uma, Sakambhari, O thou that art white in hue, O thou that art black in hue, O thou that hast slain the Asura Kaitabha”.
“Who is as Sweet as Honey, Who Subdued the Pride of the Demons Madhu and Kaitabha and destroyed them, indulging in the Din and Uproar of the Great Battle. Victory to You, the Destroyer of the Demon Mahishasura, Who has Beautiful Locks of Hair and Who is the Daughter of the Mountain”. (Mahishasuramardini Stotram).
Lord Narasimha Avatar in Sri Durga Chalisa.:
You Assumed the Form of Lord Narasimha,
And Appeared Breaking the Pillar You Saved Prahlada
And Sent Hiranyakashyapu to Heaven (as he was killed by You).
She helped Lord Vishnu taking Mohini form.
“Meanwhile, Vishnu began to meditate on Sri Lalita Mahatripurasundari, who was his very own Self”. (Brahmanda Purana: Lalitopakhyana: 2:04).
“By completely identifying himself with Maheshwari, Vishnu manifested himself in her form. She looked as if she were the very personification of the sentiment of erotica. Decorated with all kinds of ornaments, she appeared in the battlefield and brought the battle to a sudden halt." (Brahmanda Purana: Lalitopakhyana: 2:8-10).
Sri Adi Shankaracharya's description about Lord Varaha's tusks.
- The dust under your feet, Oh Goddess great,
Is like the city of the rising sun,
That removes all darkness , unfortunate,
From the mind of the poor ignorant one,
Is like the honey that flows ,
From the flower bunch of vital action,
To the slow witted one,
Is like the heap of wish giving gems,
To the poorest of men,
And is like the teeth of Lord Vishnu
In the form of Varaha,
Who brought to surface,
The mother earth,
To those drowned in this sea of birth. (Sri Saundarya Lahari).
Lord Rama worshipped her for killing Ravana.
Lalita Trishati Stotram found in the Brahmanda Purana.:
You are esteemed and worshipped by the womanhood of chastity; the Supreme of the Universe in all the stages of Birth, Growth and Destruction alike; witnessing the joyous dance of women gives you immense pleasure; You are far above the considerations of profits and losses; Mother, your instructions are always carried out and never questioned and indeed none whosoever dared to ask you questions! You are matchless in elegance and poise; you give away Siddhis with least devotion and reverence; you shower kindness with minimal efforts like melting gold is sealed in wax; Lord Shri Rama the incarnation of Vishnu performed regular pujaas to you to bestow the capacity to uproot Adharma; you save persons regardless of their deeds and fruits; you are very easily won over with selfless devotion; some times you carry plough as a weapon like Adi Sesha to punish the vicious; Lakshmi and Sharada serve you with Chaamara as a hand fan; you make exceptions to Principles of your own making some times to save situations; Devi! Several Groups of persons from varied backgrounds are unanimous in their unity in devotion to you albeit in the methodologies of their worship; you are venerated by one and all as you are the Finality of Happiness and Prosperity.
Brahma-Vaivarta Purana, Prakriti-Khanda, Chapter 1:->
The second part of Prakṛti happens to be Durgā who relieves one of all the miseries. She was adored first of all by the king Suratha. Thereafter lord Rama adored her for the killing of Ravana. 151.
She helped Lord Krishna, Lord Balrama and the Pandavas.
“The Imperishable Bhagavân Visnu will also incarnate as part, owing to the curse of Bhrigu, as the son of Vasudeva”. O Devas! I will be born, then, in the womb of Yas’odâ, in Gokula and will accomplish all the ends of the Gods. I will send Visnu from the prison to the Gokula; also I will transfer Anântâ Deva from the womb of Devakî to the womb of Rohinî. It is by my force that Krishna and Balarama would finally slay all the demons. They two will grow, day by day, by My Force and at the end of the Dvâpara Yuga, will no doubt kill the wicked kings”. (Devi Bhagawatam 4:19:34-37).
“O Devas! Now be calm and quiet and go. Let the Earth be also calm and quiet; be confident that I will certainly remove the load of this earth. I will make the above mentioned Gods as my instruments merely and I will destroy, by My own force, the Ksatriyas in the battlefield of Kuruksettra”. (Devi Bhagawatam 4:19:40-41).
“[Krishna said to Arjuna]: Cleansing thyself, O mighty-armed one, utter on the eve of the battle thy hymn to Durga for (compassing) the defeat of the foe”. (MBH 6:23:2).
“The goddess said, –‘Within a short time thou shalt conquer thy foes, O Pandava. O invincible one, yhou are Nara and thou hast Narayana (again) for aiding thee. Thou art incapable of being defeated by foes, even by the wielder of the thunderbolt himself'”. (MBH 6:23:18-19a).
Infact, There are different texts, which equates 10 mahavidyas with 10 avatars differently. But, The MundamAla Tantram, which is a ShAkta Agama gives the list of Das Mahadyas and Das Avataras as follows.:
KAli-- Krishna TArA-- RAma Bhuvaneswari-- VarAha Bhairavi-- Nrisimha DhumAvati-- VAmana Chinnamsta-- ParashurAma KamalA-- Matsya BagalAmukhi-- Kurma MAtangi-- Buddha Shodashi-- Kalki.
I hope this helps. Prd..