If you do not light a lamp while doing a prayer and reciting any mantras. But it is necessary to burn an incense stick. This is practised by Buddhist too. Why do we need to do so?
It is not completely necessary to burn incense sticks while praying. It has just become a traditionally followed practice. What is necessary is to create an environment of sweet fragrance and aroma made out of pure material (like natural products). Because that purifies the air and it helps higher beings and ethereal beings to roam there if you are calling them. It is because prāna (life air) is the food for all beings starting from animals to gods:
prāṇaṃ devā anu prāṇanti manuṣyāḥ paśavaśca ye prāṇo hi bhūtānāmāyuḥ tasmātsarvāyuṣamucyate [Tait Up - 2.3]
The gods live on breath, also men and animals. Prāna certainly is the life of all beings. Therefore prāna is called life of all.
As prāṇa acts as the food for the jivātmā, using incense sticks, dhupa, etc. through the aromatic air food is offered to them in various worships, puja and rituals. So it's not completely necessary to burn incense sticks while praying unless some god or higher being is going to come there in a subtle form. But anyway, burning incense stick is a good practice as it will at the least make the place of worship pleasant.
Usually,sandhyapuja is done at 'twilight' or 'trisandhya'. That is the time when many insects and mosquitoes are present. Along with the aroma as the other user mentioned, the dhoop/ incense stick also gets rid of the insects and mosquitoes.
This questions will definitely have many answers and the below answer is my view.
Foul smell around the environment usually brings out the negative energy and is irresistible. People usually close their nose off if they encounter some smell because it brings negative thoughts.
Try this thing, stand in a bad smelling room for about 5 mins and observe your thoughts, you will have negative thoughts flowing in.
On the other hand, good aromas bring in positive energy/ positive thoughts, so people offer the incense stick to the lord and light it up. Many people also make sure that the incense stick is taken to all the rooms in the house so that all the negative energy goes away and before spreading the aroma into the house they make sure that they offer it to the lord to bring in more positivity.
This is my view of the answer, might seem a bit illogical, but I find it true.
You might have observed that among the various things that you do during a religious function, you need to light incense sticks before the image of Gods and Goddesses. This is a major Hindu tradition that has been in existence through ages
Spiritual Reasons - There is a spiritual purpose for lighting incense sticks. It is believed to the point that the rings of smoke that ascend high into the air convey our prayers and requests to God as well as to the divine beings. The incense stick smolders itself totally and fills the air with a fine scent. This is one of the Hindu customs which symbolizes morality or excellence. It shows man the specialty of giving up oneself for the reason for others. This is the reason concerning why we light incense sticks in a puja or a religious service.
Psychological Reasons - Incense sticks are additionally used to cure a few health issues such as mild headache, depression, gloominess, etc. The fragrance spreads all around after you light an incense stick and it leaves a huge calming and healing impact on the brain. This also offers you some assistance in improving your concentration power. When you offer a prayer to God with extraordinary dedication it acts like a reflective procedure that reduces anxiety and misery.
Incense sticks are additionally used to cure a few health issues such as mild headache, depression, gloominess, etc. The fragrance spreads all around after you light an incense stick and it leaves a huge calming and healing impact on the brain. This also offers you some assistance in improving your concentration power. When you offer a prayer to God with extraordinary dedication it acts like a reflective procedure that reduces anxiety and misery.
Let me tell you the real significance of(or symbolism behind) burning incense sticks during Puja.
During Puja we say "Dhupam Aghrapayami" while waving Dhupa(incense) before God.
Its one of the 16 upacharas(Shodasha Upacharas) as well as one among the 5 Upacharas(Pancha Upacharas).So,an important Upachara, not to be missed.
The 5 Upacharas are 1)Gandham(sandal paste,scented oils etc)
Each of these 5 upacharas are believed to satiate one of the 5 Tatvas of God(Pancha Tatvas are Prithvi,Akash,Vayu,Agni & Jala).
For example, to satiate the Prithvi Tatva we offer Gandham,for Akash Tatva we offer pushpa,for Vayu Tatva Dhupam,for Agni Tatva Deepam and for Jala Tatva we offer Naivedyam.
Not only during Puja,even in Japa after Kara-Anga Nysas we recite the Dhyana Sloka of the Deity & then we offer the "Pancha Puja" using specific Mantras and Mudras(using hands and fingers).
The Mantras are as follows:
laṁ – pṛthivyātmikāyai gandhaṁ samarpayāmi|
haṁ – ākāśātmikāyai puṣpaiḥ pūjayāmi|
yaṁ – vāyvātmikāyai dhūpamāghrāpayāmi|
raṁ – agnyātmikāyai dhīpaṁ darśayāmi |
vaṁ amṛtātmikāyai amṛtaṁ mahānaivedyaṁ nivedayāmi |
लं – पृथिव्यात्मिकायैगन्धंसमर्पयामि।
हं – आकाशात्मिकायैपुष्पैःपूजयामि।
यं – वाय्वात्मिकायैधूपमाघ्रापयामि।
रं – अग्न्यात्मिकायैधीपंदर्शयामि।
Meaning-Lam in the form of Pruthvi i offer Gandham,Ham in the form of Akash i offer Pushpa(Puspaihi pujayami means worship with flowers),Yam in the form of Vayu i offer dhupam etc.
Also, note that "Lam" is the beejam for both Prithvi & Muladhara chakra,"Yam" for both Vayu & the Anahata Chakra,"Ram" for both Agni and the Manipura Chakra,"Vam" for both Varuna & the Swadhisthana Chakra & so on.So,with these mantras the devotee also satiates the corresponding Chakras in Deity .
Further significance of offering Dhupam is found in the Bhavanopanishad(a minor Upanishad allied to Atharva Veda)& is as follows:
Dhoopam – Offering perfumed Incense. Sutra 39. “Teshameva sarvadasweekaranam dhoopam” Dhoopam is perfumed incense smoke. The pleasures which our indriyas or senses give us are like its perfumed smoke which vanishes quickly. Offering Dhoopam is recognizing this and is an undertaking to control our sensory pleasures.
As far as I know incense stick stands for the principle of smell which is associated with Prithvi (earth) lodged in Muldahar Chakra. One thanks the Deity in Arati for the Pancha Bhutas (Earth, Water, fire, air and space) which makes life possible.