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 More than once I have ended up working with instructions or a document that had been superseded and I was unaware of the fact that it had been superseded.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.
2) Put paper in a file and come back in ten years and bugs & vermin have gotten to it, there's been a fire. Your paper no longer exists. I suppose acid free paper might help, but I've only heard about it, I've never seen it.
3) People have been editing paper for years. Look at all the trouble the banks go to secure checks and make sure people don't edit them.
Stinginess with privileges is kindness in disguise. -- Guide to VAX/VMS Security, Sep. 1984