If such a course has sections, the waiting list may be the result of students trying to enroll in particular sections that are full. In other cases, instructors may have set aside some seats. If you really want to take such a course, go to the first class and find out if you have a realistic chance of enrolling.
Some courses are full and have waiting lists.
If you really want to take such a course, go to the first class and find out if you have a realistic chance of enrolling.