It's not always that easy to find - even with C-h a or C-h m, etc.

But the bigger benefit is even knowing that feature X exists and there is a key combination for it. I found flash cards useful because I would get an occasional "reminder" that X exists and I should use it more often!

A trivial example: flush-lines and keep-lines. I never knew these existed till I read them somewhere. I put it in my flashcards, and over time I would remember its existence and use it more and more.

You could also try searching for "apropos" with apropos-command to find other related commands.