How do you think that "working hard" bit works and produces results?
Repetition!
If you're learning a language and trying to learn vocabulary, a new alphabet, etc, anything that involves lots of recall being necessary, spaced repetition is an excellent aid because without the basic shit you can't do any of the rest.
Plus it's supposed to be consistent over a (very long) time, not a "quick fix" so it's actually really the opposite of the "get slim quick" type schemes.
It's probably not especially useful when you're not cramming stuff/trying to build up a base to stand on, which lets you actually get on with the actual meat of the learning.