Slack is for chat which you usually care about in the moment, while Discourse is for discussion that you may want to come back to. The founder of Discourse (and Stack Overflow), Jeff Atwood, put it best while sharing an analogy from Pixar’s Inside Out on short-term vs long-term memory:
Chat is the one exception, it is the Pixar Inside Out “short term memory” stuff which is decidedly real time / immediate / connected, and you actually want to forget most of what happens in chat every day. Only the long term core memories should be pushed into Discourse.
For more, see:
EDIT: There is also a long form answer in a dedicated topic: More on Slack vs Discourse