to ensure your learners have a baseline knowledge especially subjects - where for instance 1st year students aren't all bringing an equivalent level of prior knowledge in certain areas for creating online tutorials for the more straightforward factual aspects of your subject, thus freeing up face-to-face teaching time for the more complex or contested aspects
to facilitate consolidation and revision
to cover other aspects of the curriculum that there's no time within the timetabled a part of the course for distancelearning
to allow learners to review at their own time and their own pace scale up or make more flexible training