Why is online course enrollment limited to specific times when a course is offered? Online course providers like edX and coursera offer free courses, but you can only take/enroll the courses at specific times of the year (not any time). Why is that? If you just want to learn something (not for a degree in accredited institutions), it would be much easier if you could do it online anytime.
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.