It's far more than web-based IRC and has a substantial userbase, a significant number of whom are paying for the service. Given the failures MS has had with communication tools in the past, I'm not surprised at all they mulled an $8B acquisition of Slack
Mobile clients, persistent chat history when switching clients/devices, search, @mentions notify you when you're away from your desk, and easy integrations with lots of services.
I tried to use Dropbox but it just didn't have what I wanted it to have. So, I find it to be not only replaceable, but awfully inferior to the x+y+z-style solutions.
