There seems to be four schools of thought on summer between graduation and freshman year. All four probably have merit depending on the kid, the position he plays, and the program where he's headed . As mentioned above, some schools want the guys on campus during summer taking a couple of classes and working out with teammates. Gives the players a leg up academically and gets the group together in what I assume is a much lower pressure environment yet starts to set the tone for the program. Second is for kids to play in a collegiate league that takes pre-freshman players. Gives them a taste of what baseball at the next level is going to look like, but can be a grind with a heavy schedule and often away from home. Third is to play for their travel teams on a reduced schedule or maybe play Legion with their buddies. Fourth is to stay home, work out and be rested - ready to report in great physical condition headed to fall.
I think all four have their pros and cons. For my two boys, my oldest who plays D1 got placed on a summer collegiate team. He's a pitcher and as it turned out, he experienced a little discomfort as his senior season came to a close. It was decided he give up his summer team spot and do option four - rest, be home, work out and get ready for fall. My youngest is a D3 player - he played Legion with his buddies and had a lot of fun. Probably not the best option from a baseball perspective, but kept him active on the field so it served a purpose.