Once upon a time if you wanted employees to collaborate you'd probably encourage them to use Internet Relay Chat (IRC). But about four years ago Slack appeared on the scene, and since then it's been eating IRC's lunch. That's because it's much easier to install, get up and running, and use than IRC, making it massively popular with nontechies. And thanks to a well-documented API it's easy to integrate with other programs and services. That means it's customizable and infinitely extensible, which makes it popular with developers.
But, ultimately, Slack is just a group communication tools, albeit a very nice one. And there are plenty of alternatives. In fact, many of the biggest players in the industry, from Microsoft to Google to Facebook are pushing their own tools and the number of available options is quickly growing.