We asked a D1 assistant about the prospect day experience from a coach's standpoint.
I think the benefits are obvious. You have all staff members right there watching and evaluating every single player. You are able to make a decision right then right there. Additionally, the kids are already on campus and really pumped about it. So, you can offer right after prospect day and there's a high probability that they'll accept.
From a player's standpoint if offered after a prospect day:
I think there's a mental component too. You the athlete just played against 150 kids and you were one of the few kids (maybe one or two tops) that were given an offer. That's a huge ego boost.
The administrative aspects of handling each family and making sure they all get the same attention, because they are all valuable and any of them could be potential offers. It's exhausting, but really important to the future of the program.