I probably wouldn't find it super useful on its own, but it keeps me moving through lessons and topics better than if I was just trying to learn with no direction. And you need to keep in mind whether you're actually LEARNING or just completing the lesson. Duolingo wants to give you dopamine hits by having you pass lessons, which is not necessarily compatible with learning. Learning requires struggle, which is not a good way to keep users around.
For Greek for example, it does expose me to new words/grammatical functions that I maybe haven't gotten to yet in my formal book and then I can hop over to Wiktionary to see it's formal defintion/usage/declensions/etc...
Like with any learning experience, you get what you put into it.