Stewart Butterfield had one objective when he set out to raise money for his startup last fall: a billion dollars or nothing. If he couldn’t reach a $1 billion valuation for Slack, his San Francisco business software company, he wouldn’t bother. Slack was hardly starving for cash. It was a rocket ship, with thousands of people signing up for its workplace collaboration tools each week. What Slack needed, Butterfield believed, was the cachet of the billion-dollar mark.

“Yes, it’s arbitrary because it’s a big round number,” says Butterfield, 41. “It does make a difference psychologically. One billion is better than $800 million because it’s the psychological threshold for potential customers, employees, and the press.”

Illustration by Jeremy Enecio for Fortune

Sure enough, in October—less than a year after the company released its namesake product—Slack announced the close of a $120 million round of financing. Its valuation? One billion dollars. Butterfield’s…