I agree with the words of caution here about making chat available for support (and when I joined Discourse, chat was my primary focus!)
I think it works best for banter.
Another solid use case for chat is among members who have reached a higher level of trust. It can be helpful for the helpers to have a place to support each other, as they engage more openly with the rest of the community.
One variant of that is the community team itself – using chat among the admins and moderators to coordinate amongst each other.