PLB,
My son was recruited and offered at one of these camps 6 years ago as a rising senior. My thoughts are the coaches use these specific camps to :
1) see someone again. Usually it takes more than one coach to see a recruit more than one time to get sign off on an offer. Example, RC brings in a RHP for PC/HC evaluate. Coaches agree recruit is a good fit and they offer.
2) get a diamond in the rough either athletically (previous D1 recruits) or academically that the coaches missed over the summer. Recruits that have been previously offered by D1 mid-major programs that are looking for something different or a different challenge will come to these camps. Also, potential athletic talent that had been previously overlooked by the coaches but can be further developed. However that recruit must have extraordinary academics.
The difference between Ivy recruits that have been previously offered (before these camps), and the recruits hoping to catch the coaches eyes at these prospect camps mostly lies in the support the Coach is going to provide either as a Likely Letter w/ED or just ED support. In other words, the early bird gets the worm in some cases. So, for example your son may attend one of these camps. Because the Coach has already confirmed 5 offered Likely Letters recruits prior to the camp, he'll be looking for another 3 recruits that are willing to apply ED with his support but no Likely Letter. Again, the early bird gets the Likely Letter worm. The ED route with no Likely Letter can be a little more risky.
I hope that helps. JMO.