But what if you do need to collaborate on pieces constantly? Bouncing emails back and forth isn't the best. If you don't have an office environment that's conducive to collaborating like that, slack is also a boon here. Doubly so if members of the team are distributed. What if you've got issues with your email where there's a ton of it, and 90% of it is only mildly important? If lots of the people sending out that email instead just sent chats to a channel that you turn off notifications for, you won't see notifications for it and your inbox is cleaner.
Of course it can be disastrous, with channels turning into nothing but a bunch of GIF sharing and other stupid shit. The onus is on the team to keep work channels clean and focused, and have appropriate channels for goofy stuff that people will naturally tend to participate in.
Note that I'm not saying whether or not this ultimately allows work to happen more efficiently. I honestly can't tell if using Slack makes things easier in the long run, but I certainly prefer using it over email if I can.