It breaks up a large task, learning a language, into smaller bite-sized chunks. It rewards users with coins that can be used to do power-ups. It turns learning into a rewarding game.
Perhaps this principle can be applied to other domains as well.
You can even take this further with reading books. Sure, you can get a star by reading a book for like an hour a day...if you're using Apple's iBook or whatever. That just means you're locked out of other books not on that platform or books you have in other unsupported formats or gasp physical books.
https://en.wikipedia.org/wiki/Spaced_repetition
My friends that are in med school all use spaced-repetition to cram information into their memory banks, and the app they use (Anki) lets users share flashcard packs. Incidentally, aside from medschool-related packs, there's also packs for learning most languages. I don't think you can ever learn a language through spaced repetition, but it's certainly useful for expanding your vocabulary (learning new words). I started out with Duolingo but switched to Anki because I'm learning a tonal language which doesn't quite sound right with Duolingo's TTS (and the Anki card packs are accompanied with recordings of real speakers).
..and yet I still pop into Duolingo every day though to get my good-boy points :)))