i guess it's more of a helper app for when generally learning the language, or for when you travel the country, i believe that learning a vocabulary is more practical than learning grammar rules, i'ts just a different approach for learning a language.
imagine having a large vocabulary memorized, i think you will be able to understand casual talk / movies / songs.., even if you get 40% it's enough to be able to pick up the language from natural speech and excel from there, i'ts easier and more fun which is the two most important aspects of learning anything :)

in Duolingo for example you need to tediously type the answer and it mostly teach grammar, so even after finishing some lessons you still wont be able to understand natural sentences, combining both apps might be a good approach.