What are the main reasons. of putting a hat on babies ?

Prevents heat loss: A baby's head is a lot of surface area for him. Thus, he can lose heat as the room air cools his head. Wearing a hat keeps his head a bit warmer. That said, if he doesn't wear a hat, nothing bad should result. If a bit cold, a baby will burn fat to generate heat, and therefore will need to eat more or won't grow as fast. No hat needed when hot. And vice versa, don't let a baby get too cold.