re: Why do people send kids to boarding school

There were several boarding schools very close to where I grew up. Lawrenceville Prep, Peddie Prep, The Hun School, Princeton Day School, and etc.. They weren't prisons made for children by any means because they had basically everything a school aged child could ever want. For the most part high school aged children involved in sports, clubs, and activities are barely home during the weekday. I know very often I didn't get home from school (because of school activities) until after 9pm. By often I mean 2 months straight.
As people already mentioned they are expensive. $50k or more per year. The education is high level, and the social circles are elite.
You still see your family often, and it's more like an extended sleep over. I really don't know many people who ever regretted the experience, and I don't know parents who regret it either.
If a family can afford it it's a great experience.