If your job requires that you spend a lot of time communicating with people across organizational boundaries, email is perfect. Email is the lowest common denominator, and it's going to cross organizational boundaries really well.

One advantage that I think Slack has for most people who use it is, you pull out your phone, you look at the home screen, there's the Slack icon. You know when you tap this one, it's all the people you work with, and it's only the people you work with. And that's a big advantage.

There are two big benefits from moving conversations from a mode where you're addressing individuals, or groups of individuals, to addressing a channel which is a topic, a project, a functional discipline, or whatever.

Stewart Butterfield Quotes