That's how you get bloated budgets. Why spend thousands of dollars per year on a communications platform if you don't need to spend it?
IRC, mattermost, email, Gitlab, Wekan, Skype/Google Hangouts/BigBlueButton.... there are plenty of tools to communicate without spending loads of cash.
None of those tools work as well as Slack out of the box. Some of them require significant work to integrate with other web services. That's time your staff is spending on those projects rather than your core product.
Seriously, Slack was the EASIEST things to justify as an expense when we proposed it to upper management earlier this year. And they are known to be very stingy.
why use macs (or Dells) instead of a cheap china-built off-brand PC? Why use fancy Pilot G2s instead of 10/$1 bic pens? Why use ${SuperiorProduct} instead of ${InferiorProduct}? Because it has value outside of the immediate short-term economic value.
Of course, it is always a matter of perceived utility. I buy really nice food sometimes, because my happiness gained from the delicious steak is worth it.
I might hypothetically use an open source tool instead of Slack because it's not worth $2000 of productivity for 15 people per year.
