What does the cap on top of the chimney really do besides not to let rain and snow in.

It will also help to stop backdrafts. The wind can't push down the chimney air column and force back smoke into the room.

It also deflects debris from trees and birds. Not all chimneys have the "can"

Thanks to both of you for your answers.

The cap can be closed when not in use preventing cold air from entering into the room .