It doesn't help you understand stuff, and it isn't suppose to help you to understand thing. Flashcards are purely for memorization. Obviously you need to understand what you are doing, but some times memorizing is important.

If you memorize a formula and all its variations, then whatever you need it or see it, you can quickly use it. On the other hand if you do not memorize the formula you will have to derive or prove it every time you see it, and you will have to do it for all its variations as well. Which is far slower. Which is why if you are looking for speed it takes memorizing stuff as well.