Quizlet is great if you're looking for pre-built decks, as you mentioned. As far as I'm aware, there's no spaced repetition feature, which is the main value in Anki.

Also, one of the values to me in using Anki is creating the cards myself. It allows me to mull things over and decide what part of a fact is important, and how I'd like to recall it.

Relatedly, the value isn't necessarily in reading someone else's study guide before a test, it's in creating your own study guide. That process helps you understand and retain the material far better.