We forget quickly. Without repetition, much of what we learn in a course is gone after a few weeks. Spaced repetition counters this by repeating the material just before you would forget it, for example after a day, a week, and a month.
Each repetition can be short. A couple of quiz questions are enough to bring the knowledge back, and it is precisely the act of recalling that makes the memory stronger.
The method suits rules and procedures that need to become second nature, and compliance, where knowledge must be kept up over time.