In all subjects, except maybe math on some level, there are some basic facts that you just have to memorize and you can't derive the answer from the other facts you know, so it makes sense.

Even if you could theoretically derive an answer it's often still good to have that work precached. If you read a complicated scientific paper you need to have a lot of knowledge precached and can't spend time to derive facts from other facts.

It has to go hand in hand with trying to understand the subject at hand though, otherwise it would be just trying to guess the teacher's password.

Yes. That one of the reasons why it's usually better to make your own cards than to use the deck that someone else created.

Wozniak's rule number one: "Do not learn if you do not understand" and rule two: "Learn before you memorize".

Theoretically I think that it's possible to create good decks that other people can use, but it's no easy project.