While in my experience, PMs might take a while...

But they are indeed great for secretkeeping, revealing secrets, and keeping things organized. If the party splits, PMs can allow the DM to decide when the players meet back up with each other that make sense to the game. Plus, there aren't haphazard IC posts if everyone goes their separate ways; There would just be one long post to encapsulate everything that happens during the time the party is split. Less of a chance of getting confused on what's happening in game.