Our teams are entirely remote, and slack fills in for some of the things we miss by not being in the same room. Async tech conversations, status and reachability, crisis coordination, a feed from various other systems, and some banter. I actually like that the free tier deletes messages; we have other tools (wiki, bug tracker) to serve as a permanent record, so everyone is clear that anything that needs to be documented should be written up elsewhere. If it didn't exist, irc would for us almost fully replace it; it's just a slightly nicer ux.