What’s funny is, we don’t usually consider a thing to be “learned” if we haven’t retained it, so in a way we have to reduce the bar something has to reach to be considered learned to then find value in using spaced repetition. The classic example might be reading a book or article, feeling like you’ve gained some new understanding, but only retaining some of the core ideas, or only being able to recall the core ideas from certain mental entry points that happened to stick around in your brain longer than others. I think the real win of spaced repetition is being able to “choose what you remember” from things you learn, and not relying on your brain and whatever subset of that learning it chooses to retain without assistance.