Maybe if you're billing with a paper and pencil, mailing out invoices, and you have to drive a bag of checks over to the bank.
BuildKite is an orchestration SaaS, they don't even own or run the hardware the agents are on. If it costs them $9/mo/user to manage that most of the C-suite should be fired.
You'd be surprised how many firms have subscriptions to billing insights, know-your-customer, fraud detection and other similar services. Many of the "AI" companies that are emerging do such things.
I would have thought those sorts of people just go for the $0/mo option here.