I'm talking about using slack as a member of various groups, with very different focuses. The fact that it is centralized means I don't need to have Yet Another Server I have to remember the address to, the username, the password, etc, and enter in on my desktop, my laptop, my mobile, etc. Plus, if one of those decentralized instances has a mobile app, chances are, I'd have to put in configuration info there for each and every service. Self-hosted communications services quickly become a pain in the ass when you end up needing to deal with more than a couple, and sometimes centralizing things a bit is an actual benefit.