The time limit is enforced discipline. It's arguably their main benefit. Some people study without an online course... most don't.

I don't have data on completion data on timed vs untimed courses. The former's popularity is suggestive, but confounded by marketing, more people online, prestige, packaging of courses, lecturer/tutor availability, cohort forum, etc.

But I do have one anecdatum: I took Ullman's Automata, even though all the lectures were already on youtube, and slides, tests and solutions on his homepage. I found the schedule very tough towards the end... I meant to later review some parts I needed more time on, but over a year later, I still haven't...