And why should I use it in place of email or the telephone?
Because keeping up with a dozen team members in geographically, time zone and time schedule diverse places is a pain. More than once I have ended up working with instructions or a document that had been superseded and I was unaware of the fact that it had been superseded.
You end up in a situation with a dozen people hitting "reply all" or calling each other on the telephone leaving messages. While you're listening to one message or talking to one person, three others call and leave more messages and sending emails in between.
Maybe Slack isn't "The" answer, maybe it still needs some work, but it's a step in the right direction.
Why did the Roman Empire collapse? What is the Latin for office automation?