Knowledge that is primarily conceptual (like almost all of math) generally does not benefit from spaced repetition. The learning involved is understanding -- a new concept may be hard to understand in the first place, but once you get it, you don't really forget it. Or you just need a super-quick refresher if you haven't touched it for a few months.
While knowledge that is primarily arbitrary-factual is the perfect candidate for spaced repetition -- mainly things like vocabulary, medical terms, and so forth. Just associating a largely arbitrary name for something. And indeed they are mostly useful for learning for exams. E.g. I used it to learn Chinese characters and could never have passed Chinese class otherwise. But on the other hand when I actually lived in another country that speaks a different language, spaced repetition isn't much of a help -- you learn vocab just by absorbing it day-to-day, like a kid does.