So many times I've searched some weird intranet hostname, or $megacorp_specific_lingo and found it in Slack, yet nowhere in our Confluence. Confluence is great for more long-form stuff, but it also gets stale. But the great thing with a slack message, it's mostly immutable, so I can see the date (a decent proxy for relevance). I don't have to peer through the edit history in Confluence to ascertain the age of the information on a document with a long lifespan and many edits.