I use Zulip instead of Slack, Mattermost, or RocketChat because of its first class threading. One week after switching to Gmail (in 2004) I realized I was never (willingly) going to use an unthreaded email product again. I had that same experience the first time I saw Zulip.

Zulip is also fully open-source, with a well-maintained (e.g. 90+% test coverage, fully static python), easily extensible code-base. In many companies, your communication platform (chat or email) is the center of the workplace -- no one asks for a chat integration into their calendar, they ask for a calendar integration into their chat. A fully open-source codebase means you can customize Zulip to your needs, and are never at the whim of a corporate maintainer who can't or won't fix simple bugs, or who will charge you tens of thousands of dollars for making minor customizations.