When it goes well you get live, interactive support and get your question answered fairly quickly. Nice and convenient. But as you’ve said already, it has drawbacks and it’s where forums and things like Lemmy come in, where sometimes you can get replies days later.
They’re different systems that reach different audiences. You use whichever based on the needs and complexity. What sucks is when the chat rooms develop some knowledge that doesn’t get known outside and it’s also not indexed anywhere on the web. Some things are better discussed in forum format (or mailing lists if you’re very oldschool), while others are just better interactively and the back and forth on a public forum would just be painful.
Usually there’s a bit of an overlap at least, where users are usually in Discord/Matrix/IRC and some forum or reddit or fediverse community at the same time.