The company could only use cash to do dividends or share buybacks, and the company is worth more than its cash on hand (maybe 10-20x as much more in Slack’s cas

The company could only use cash to do dividends or share buybacks, and the company is worth more than its cash on hand (maybe 10-20x as much more in Slack’s case.) A public offering makes it possible for effectively every shareholder to sell their shares for the full value without the company having to make a strategic financial decision to return cash to shareholders.