Customers start out by wanting all the flexibility (for their devs, usually) to spin up their owner servers, and want used-based billing. It would be a waste to pay more, no?
Then their own book keeping department yells at them, because they use SAP, and every bill that differs from the contract or the previous month is sheer hell and requires 5 levels of approval / justification / whatever.
So the next iteration is that they buy a fixed contingent of "cloud points" per month so that the invoice remains constant, and there's reporting and/or a firm (but often unwritten) promise from the account manager that they'll notify the customer if/when they ever run the risk of exceeding their contingent.
In really big / inflexible companies, it really seems to be easier for the purchasing managers to justify higher but constant prices than variable prices. The inefficiency boggles the mind.