Unpopular opinion: I don't find them useful. They are enormously addictive, which isn't a bad thing if this leads to good results. What I found is that they create the illusion of results and false achievements ("Yay! You're intermediate now!") when in fact, they don't get you significantly nearer to achieving your goal of understanding and speaking the target language. That was my experience at least.
I used one popular app daily during my commute when I was first attempting to learn Russian. By the end of the course/challenge (I cannot remember what it's called), I had convinced myself I was at a lower-intermediate level and the app seemed to confirm this. What a joke! I couldn't understand even simple Russian videos and songs, couldn't read at anywhere near a normal pace, and when I had my first conversation with a teacher, I could barely understand or say a word.
It's mostly a grammar-translation approach (so, nothing revolutionary or firmly grounded in anything evidence-based) and one in which you select or re-arrange words, so the results aren't surprising. I don't recommend them. In my opinion, the time would be better spent getting a solid foundation so that you can read and understand simple sentences in the same time frame. Ideally, whichever approach you use should include an evaluation of your ability to understand gradually more challenging sentences and communicate in a way that the people listening can understand.