Most non-free software is marketed to companies these days, except for games. Most of it is to support putting non-software products in front of people.

So in that framing, a company can pay for recurring prices for software, and this benefits (obviously) the seller. It benefits the buyer because no software is useful forever. If a company makes a very expensive one-time purchase of Slack, they may have wasted their money if they get acquired and everyone is moved to Teams instead. So it makes sense for a company like Slack to charge a "low" price for each user per month.

And no individual consumer would purchase slack for 9-18 dollars per month. No, they are interested in buying a new toilet seat - that can only exist because large companies making toilet seats needing software like slack to internally discuss if they should add padding to all their toilet seats.