Many have encountered people who ask why do one offer Prasada. But there's really no reason why. Consider this. If we see a bird building a nest in accordance to nature, would we ask why? No. If we see a happy person laugh and leap, would why ask why. No? Because it is obvious! It is obvious the a happy person would giggle or smile and a bird would build a nest as per it's nature.

One offers prasad due to bhakti. Despite Bhagwan is formless and never asks for much, we still want to serve him due to our love and bhavya. Meerabai offered prasad to Shree Krishna, and Shabri offered berries to Shree Ram. Sudama gave Shree Krishna pawra. All offered without the deities asking.
We all get immersed in the love of the deity. Such supreme love takes us in a divine trance.

Also. Within darshan time, Prabhu is our guest. We must lovingly give food to our guests and please Him. He may be a guess in our material homes but a resident in our hearts!

Why do the divine descend on earth on their birthdays.

The birth/jubilee or Jayanti of the deity is celebrated as their darshan day, as it is known that they come down to give blessings to their beloved devotees. This is the same way as if one of us is celebrating our birthday. We would want to celebrate with our closest and most special ones. Bhagwan is so bounded by us, that He would leave Swarga to be with us. And we're so bounded by Him, that we seek every drip of his blessings like precious amrit.