To be fair, why he uses Slack is likely due to his employer using Slack.
Re:Slack Says It's Filed to Go Public
by quickthrower2 on Tuesday February 05, 2019 @03:26AM
(#19083741)
And the reason his employer uses it is to ensure everyone can interrupt everyone and give each other more work.
Is there a competitor that doesn't have the emojis/gifs/videos? They're easy enough that everyone adds them, so I don't think we can conclude "the market" has made a clear choice.
(Ok, I remember HipChat didn't have gifs but there was a lot else wrong with HipChat)
It's why you use Slack, rather than why people use Slack. I'm sure you could make do without the emojis, GIFs, and videos, but the market says people seem to think otherwise.