>> Slack will also let employees and other shareholders sell equity to investors as part of the deal

Translation: partial liquidity for founders and early employees.

There was an interesting article with the CEO which adds a little color: https://bits.blogs.nytimes.com/2015/04/16/is-slack-really-wo...

Only a guess, but to let early investors cash out and then use the rest to finance their operating losses?

They don't. But there is too much money seeking "unicorns", so you might as well vacuum it up.

If you suck in $250 million and have only 500 employees, you can cruise for a very long time.

Raise money now on much more preferable terms when you don't need the money, store it in a war chest, improve morale/buy-in with "money on the table" for your employees/early investors.

It's pure win for Slack right now.

Deep pocketed competitors entering the space would be another answer.