In my own experience using spaced repetition for math: math has both semantic and procedural knowledge. The procedural knowledge comes from doing problems and rewriting proofs. But the semantic knowledge is also important, and you can acquire and retain this through spaced repetition.

I was going to write some rules specifically about math but I might write those as a separate post because they got too long. I think I've benefited specially from memorizing the proofs of theorems, though refactoring proofs into multiple lemmas to make each proof small enough to fit in a flashcard is a tedious process.