One of the biggest reasons to go to the PG National Underclass in December is to work on getting invited to the PG National Showcase in June. That's the first big event after the draft, and it is kind of the kickoff event for the summer when kids stop being juniors and start being rising seniors.
PG National in June is by invitation only, and is one big recruiting free-for-all. While my own son was an early commit, his friend/teammate went to PG National while holding an offer or two but not yet committed to anyone. In the three weeks after attending (and doing quite well there as both a pitcher and a hitter), he received over 20 offers. No, I am not kidding. Several schools contacted him by saying, "We really want you to come see what we have to offer, and to let you know we mean business, we are willing to offer you ____ right now even before you come."
But bear in mind, only a few hundred end up getting invited to PG National, so if you don't think you're in that realm, this may not apply to you. The boy I'm talking about was ranked I think # 126 in PGX's final rankings for the class and was taken in the pro draft as well. I realize not everyone is so fortunate. But if you think you might be in that class, it does go to show why PGNU and PGN are worthwhile. Actually I think this kid I'm talking about was ranked around 380 or something prior to his junior year season, then maybe around 250 going into PG National, so he made the most of his opportunity to be seen not only by PG's evaluators but also by pro scouts and major D-1's from all over the U.S.
But you have to get there first, and PG Underclass is probably the best way to take your shot at getting there.