Not having to maintain your own server. Slightly better integration with other things (e.g. messages sent when you're offline can be emailed to you) - nothing you couldn't do with a bunch of client plugins, but it's there and set up for you already. A very nicely polished in-browser client, nicer even than irccloud, which makes it much easier for non-technical folks to use.
There are lots of open IRC servers on which you can create password protected rooms. That plus Google Docs /Drive works great for small companies. For larger companies, however, I can see why something like Hipchat could be useful.
