What is so special about Slack?

I am not a big user of Slack but after a friend told me yesterday that he dropped completely emails in favour of slack in his startup, I have been thinking what is so special about slack.
At the end, it seems to me that it is basically similar to emails that people classify in folders but exposing these folders directly to everyone. So the classification is done by the sender rather than the recipient. And the recipient can easily opt-in/opt-out from specific email topic.
My friend also mentioned good integration with other system such as JIRA. Something I need to look into.