Not really.

E.g. The firms investing $250 million basically would like to see ~3x return or more on their money. The want their $250 million to turn into ~$750 million. That's the whole point of investing.

If Slack simply parked $250 million in the bank for 1% interest rates, that's something Softbank could simply do themselves. If Softbank can buy $250 million treasury bills for 1% return, there's no need to give Slack $250 million to do it for them. Besides, VCs like Softbank can't survive on 1% returns; they're hoping for ~20%+ returns.

Instead, Slack is supposed to put that $250m to productive use and make the company more valuable. This makes more money for everyone including investors. (This doesn't mean Slack spends the $250m all at once though. Softbank and CEO Stewart Butterfield probably had long discussions on the strategic uses of the potential capital. Presumably, Softbank agreed with Butterfield's vision which is why they invested.)