Are they graded? Is it for a teacher to get to know you?
CK would say, “For ruining your summer.”
Mostly, schools suggest summer reading to keep you in practice and to give classes a common place to begin the academic conversation in the fall. I don’t believe Choate ever quizzed or graded on summer reading (that CK ever reported), but I have heard of some schools that do check to make sure at least the required reading was completed.
At my kid’s school, there is a required book discussion group during their first week back at school for one of the summer reads, and they have to be prepared to write essays on the others (I think they are required to read at least 3 or 4 of the books on their list).
At my kid’s school, it’s up to the individual English teacher, but they always seem to do SOMETHING-- maybe a test on the book everyone in the grade has to read, then an essay test or paper on the 3 chosen from a big list.