I found Slack rather poor in explaining what they are doing. This text is basically their entire landing page.

"When your team needs to kick off a project, hire a new employee, deploy some code, review a sales contract, finalize next year's budget, measure an A/B test, plan your next office opening, and more, Slack has you covered."

Do they offer A/B testing? HR tools? Code deployment? Who would have guessed it is chat.

Their /features page does a better job: "It simplifies communication. Slack brings all your team's communication together, giving everyone a shared workspace where conversations are organized and accessible."