> Mainframes were rented back in the day, you paid by resources consumed

To expand on that, IBM used to rent mainframes based on a 40-hour week. The computer had a usage meter (like a car odometer) that would keep track of how much time the computer was running. If the meter ran over, you would be billed for the excess charge.

The computer actually had two usage meters, with a key to select between them. When an IBM service engineer maintained the system, they used their key to switch from the customer meter to the maintenance meter. Thus, customers weren't charged for the computer time during maintenance.

One interesting thing I noticed about the IBM 1401 is that it's built from small circuit boards (SMS cards) that are easily pulled out of the backplane for replacement. Except the cards driving the usage meter. Those cards are riveted into place so they can't be removed. Apparently some customers discovered that they could save money by pulling out the right cards and disabling the meter.