Duolingo is great, because learning a language (and many other skills) is a matter of doing it consistently. If you have a 4 hour class once a week but don't study or read anything in Spanish outside of the class, you will forget things before the next class.

Of course, Duolingo is not magical, but it gives you enough vocabulary and understanding that you can start following people on twitter/instagram and know what's happening. Then you start trying to reply and interact, and then at some point try a book, then a TV show..

Also, many people plan on starting something (like learning a language) later on, when they have more time. Many people I know that wanted to learn English (Im Brazilian) didn't start years ago because they `didn't have time`, so now, after a few years, they still need to start from 0