This may seem absurd to the modern mind, but actually it serves a very useful purpose. Crows and dogs can see spirits which is why they are associated with Yamaraj , the God of Death. When their favourite food is laid out, a nervous spirit who is still there, will hover around it and the crows will leave it alone. If the crows do not eat the food, the pandits will assume that the spirit is still not satisfied, and will perform a few more rites to help the deceased to move on. When after-death ceremonies are properly performed, there are less ghosts left to cling to the physical world.
Cremation also helps the dead to depart.
It is easier to cut physical ties when there is no body. Also the mantras chanted during cremation are powerful and help the dead make a clean and easy departure. When the body is buried, the dead are confused by the ties they feel to it, and it is so much harder to leave. The living may hinder this process by acting as if the person is still there, by visiting the grave often. The living think of the grave as the resting place of the dead, which it never is.