At least in software development, SO is/was very useful, as they provide a curated repository of Q/A where the answers follow the current state of tech.
Every time I started out learning a new framework, SO would be tremendously helpful, because in software development, you mostly have questions that have correct and incorrect answers.
I said SO but I should have said SE in my case - it just doesn't work for research fields because these especially are all about exploring unknowns, yet SE doesn't allow this...
Maybe software engineering is all solved and there's no more ambiguity left to be discussed.
Why would anyone other than undergraduate students spend their time on such a site