Why do Prep schools ask other schools you're applying to?

PS - I did not mean “Reach” to a NE school as a Reach - what i meant to say was “giggles”, “difference”, or “what the heck let me apply to PEA to see what happens even though my folks want me to stay in British Columbia”. I meant no offense to West Coast Schools (BTW - most amazing school we saw was on Vancouver Island…but no FA to USA kids at the time).