I posted a lot of the benefits in my first post.
It contains a lot of hormones form your pregnancy, so when you loose your placenta, your body is kind of jolted and that can sometimes cause ppd(from my understanding, there are other factors too of course). Mammals eat their placentas not only to get rid of evidence the birth, but also because it helps to slow the bleeding, brings in milk sooner, and helps the hormonal imbalance that often happens after birthing.
I have someone who lives in my town that slices the placenta into thin sliced, dehydrates it, crushes it to a powder, then capsulizes it. I never have to do anything with it myself, and then I take it in capsule for, just like a vitamin or any other supplement. I truly believe that it saved me from horrible depression last time. With my third baby I was so bad off that I would refuse to nurse or hold him, and we all have lingering emotional scars from that.
I know it seems weird, and kind of gross at first, but there is a lot of info out there on the benefits, do a search on MDC. I know there was a huge thread about it a while back. I'll look for it.