Memorizing tells you what. Asking why lets you rebuild it when you forget.

A memorised fact is a single stored item with no supporting structure, so when it goes, it goes completely. Understanding why something is the case stores the reasoning instead, which means you can regenerate the fact even after forgetting it. One is a copy; the other is the machine that makes copies.

After learning something, ask what it depends on and what would break it. Try deriving it from scratch a day later without looking. Where you cannot, you have found the place where you memorised instead of understood.