why do you use leaves to cover it.

Got it!
This is like a tamale in a corn husk, or recipes using the 'en papillote' technique -- did you ever wrap vegetables in foil and put them on a grill, or put a foil wrapped potato in a campfire? Here's information, including leaf information.
http://en.wikipedia.org/wiki/En_papillote

I think this was for the previous question she/he asked http://www.food52.com/hotline/13180

Well what kind of leaves and cover what. My cat uses leaves to cover his but I do not think that is what your asking