Earlier quoted context omitted.
Those core, must have features right? "Oh no I can't use that business chat tool, it doesn't support smiley faces and cat reaction gifs".
Maybe relevant wasn't the right word. But it's what makes slack slack, over say, irc.
The key thing Slack offers most 'customers' (even those who don't pay) compared to something like IRC is the simple setup/use.
I say this as someone who hates Slack, but has had to endure it on a number of client projects.