I met Slack while I was getting into Zapier, while looking through all the things it could automate. Our team was using Skype to do all the main communications, but there’s no open API to enable automation. However Slack had an API and Zapier could pipe any information from other apps with ease. We had GIT alerts posted to a separate channel, and whoever was interested in being informed could get notifications.

It’s important to keep teams informed, or you’ll be wondering why you don’t nip issues in the bud, or problems brew in the background while team members are kept in the dark. Slack, Skype, and Discord are all ways of doing this (and all free to a point).
Personal experience
While Skype ruled for communications (old habits die hard!), Slack will scale well, and with Zapier hooked up will enable a business to be fluid and agile.
Use cases
When you need to keep your team informed, and a place to share files.
Cons
With the prevalence of Skype for business, Slack isn’t on a lot of peoples radar to enable a team to move into it effectively.