Some schools do it because those students’ stats don’t go into the USNWR calculations (that’s how Northeastern climbed the ranks); they can accept someone who is clearly qualified and who might have a test score that is out of line with other parts of the application, etc.
Some may do it to spread out the administration duties; if they have to process 9000 new students, it helps to do some in the spring and summer and the bulk in the fall. Florida schools do it because everyone needs to do at least one summer session and some like to start and get it over with, and it keeps people on the campus in the summer (full dorms, classrooms, dining halls) which keeps staff employed too.