There are as many breast feeding benefits as there are babies...here are just a few:
These antibodies are in very high concentration the first couple days during the colostrum stage. There's no other way to offer the baby this level of protection when they're newborns. (Well...maybe a plastic bubble but that's just not practical :)
I'm no physician, but it would follow that breast feeding may help prevent or shorten symptoms of postpartum depression.