Why is online course enrollment limited to specific times when a course is offered?
Actually for both of them some courses are fully autonomous "anytime" courses, while others are restricted to being offered at particular times.  So far as I can tell, the time-restricted courses derive two benefits from being time-restricted:


It creates a "cohort" of students learning similar things and doing similar assignments at the same time, who can then help one another in the associated forums.
Resources needed to support the course (e.g., TA monitoring, course material updates) can track the progress of the students through the course, increasing the return on resources and minimizing disruption to students.