Slack is kind of like an internal chat system. By removing 3rd party communication from the platform, it reduces outside distractions and the chance of accidentally sending something to a client. It's also much easier to read a Slack thread than to follow a long, multiple recipient email conversation.
Slack also reduces the clutter that is endemic with email. What percentage of your emails are actually important, or to set the bar lower, even worth reading? I bet it's less than 10%....probably less than 5%. Yet it still requires a few moments to review each email and separate the wheat from the chaff. That's just an inefficient use of staff time, and Slack eliminates that waste.