Why do colleges use a waitlist?
Colleges have a limited number of spots available each year and a degree of uncertainty about how many students who got accepted will actually attend. If a significant number of students that a college accepts also got acceptance offers from other colleges, the college has no way of knowing for sure how many accepted students will choose them.
The waitlist helps colleges keep students on reserve. Once available slots fill up, and students start declining acceptance offers, the college admissions committee can begin moving on to the waitlisted students.