Why do colleges "Wait List" some students?
The answer is found in the principle that colleges are selling higher education services, and parents are the customers. That means that Letters of Acceptance are issued primarily for the benefit of the colleges. They accept who they want, when they want, how they want.
Colleges track their admissions data very closely, over decades of time. Those large numbers give them a very accurate picture of how many letters of acceptance are needed in order to fill every seat available. Remember, each seat in a first-year cohort is worth hundreds of thousands of dollars over the course of the student's education.
In the administrative offices, "yield" is the number of students who actually matriculate from among the greater number to whom letters of acceptance were sent. Let's take some easy numbers to help your understanding.
Big Time College plans for a first-year class of 2,000. Their historic yield is 50%. Therefore, they sent out 4,000 letters of acceptance, and notify another 1,000 they are on the Wait List. Suppose this year they miss their yield by 1%; 20 students. Big Time College's wait list is now accessed. Starting at the top, they go down the list until they find the 20 students who will accept an offer of admission.
What is the top? The wait listed students are ranked academically from most-well-qualified to less-well-qualified. Those at the top of the list probably missed by inches a letter of acceptance in the first place.
Should you wait on the wait list? Probably not.
You will never know how close to the top you are.
You will not find out if you are chosen until the deadline for commitment has passed for the other colleges that accepted you.