This sounds a lot like why horse mares will eat the placenta from their foals after giving birth. Foals are born able to run, but they're nowhere near as strong as a mature horse.

I was under the impression they ate it because of all the vital nutrients that the placenta had, but I dont think anyone had conclusive evidence except in an evolutionary biology hand-wavy way.

If that were the case we should expect health problems from horses that go too long between pregnancies, yes? Otherwise the nutrients are not "vital."

Whence do you suppose the nutrients in a placenta originate?

Actually everything in this thread is a just-so story. It's pretty silly to pick out one specific rare behavior as even relevant to evolutionary fitness. How would any of this be tested?

I dunno, if you have been gestating a foal, you might be a little undernourished at the end of the process, so a placenta's worth of nutrients might then be "vital."