I don’t think much in this is specific to Discord so much as it is to chat/IM in general. Honestly we use both chat (yes via Discord although I’d love to move to Matrix) and forums. They just serve completely different roles. Traditional style forums (whatever it is, Discourse, Flarum, Github Discussions) work really well for “long form” topics and asynchronous conversations. i.e. if there is something to discuss that is complex and can attract valid conversation over the course of days/weeks/months then it is ideal.
Chat on the other hand is great for co-ordinating and asking quick one-off questions that will get you an answer really quickly. We use it all the time to just discuss general plans, ideas etc. and answer simple questions like “how do I do x?”.
I think most of the (justified) hatred is to those projects that only have a community via chat which is valid - on big projects it can be somewhat difficult to get a word in and get noticed if you have a “simple” question which wouldn’t be a problem on a forum.
The moment I see the same question popping up more than a couple of times is an indication that it should be documented by somewhere that is actually indexed by search engines, normally the website/faq/docs/wiki as it is clear there is something missing.
To me, as part of a small team/project, it feels so much better to be able to use chat for every day communication just as I would at work. It allows a lot more expression in communication than forum posting. It has really helped us have a good sense of community and teamwork we might have not otherwise had.