Comment by gspetr

Andreessen Horowitz was a major backer of Slack's predecessor, Tiny Speck, which was originally building a game called Glitch.

When Glitch failed in 2012, founder Stewart Butterfield offered to return the remaining $6 million to investors. Ben Horowitz instead encouraged Butterfield to pivot and build out the internal communication tool the team had developed for themselves, which eventually became Slack.

I saw an interview (don't have the link at hand unfortunately) where Horowitz said he didn't much care for the $6M as he had already been set at that point moneywise, and essentially wanted to gamble on an off chance Slack succeeds.

Horowitz continued to support the company through its rapid growth and eventual direct public offering (DPO) in 2019.