I think Swampboy does a good job of covering why one would and would not attend a camp. I'll add one other situation where it would make sense to go.
Some colleges have camps where they invite a large number of coaches from other schools to "work" the camp. My son attended one such camp where there were more than 20 coaches in attendance from schools ranging from JUCO to Top 10 D1. Son had no illusion that the school hosting the camp was interested in him, but by attending he did catch the eye of a school that had not seen him before. That school ended up in his final three.
Interestingly, son is working his college's camp today. He went to that camp a year ago during his senior year of high school because the coaches seemed seriously interested in him. After the camp, they had no interest whatsoever. Six months later, after a good senior season, the coaches changed their mind and offered him. You never know.