One of the nice things about slack (and it's alternatives) is that it is really easy to hop in a channel and see what is happening in a part of the company. Lots of times I find a bug or issue with an internal tool, and go to the related channel, and right away I can see they noticed the issue and are working on it. With email you don't have that kind of visibility unless you are sending company-wide emails every time there is an issue with some tool or system, and that doesn't scale very well.

Also it's nice to be able to search through history for things like error messages, or just keywords related to what I am working on. Plenty of times I have just searched for an error and found the solution right away, also not possible with email unless every single time you discussed an error message, those emails were sent to everyone (and then new hires still won't have access to data before they were hired).