Then why are you using Duolingo in the first place?
That app has to be the worst method, because people think they are progressing while they in fact don't. From personal experience it's the worst method I tired to learn a language, and the content present for languages I know isn't good either.
1. If you can't afford something better. I keep getting pummeled on the leaderboard by people doing 5000+ XP/week worth of English for Spanish Speakers. None of them have Super Duolingo (you can see if they have it by looking at their profile)
2. If you're testing the waters. I already talked about the money, but a semester of a language class is a big time commitment, too. You also lose scheduling flexibility that's especially desirable when you don't know if you want to commit to a class yet.
Then there's supplemental, extracurricular practice. If you never use a language outside of class, you'll lose it very quickly. Anyone who had to take a language class in middle/high school knows this. Duolingo (or any other app) can help reinforce learning, by regularly poking the "foreign language" part of your brain, even on days when you aren't in class.
The reason I use Duolingo because it's the only thing I'll actually use, thanks to the emotional blackmail.