Many companies lease hardware, not so much for the hardware obsolescence, but for the accounting & governance. A lease is a series of one time expenses (like a utility) that arrive in manageable amounts and are easily directed to a business units annual budget. Purchasing computers is a capital outlay (like a new building), requires upfront funding, which in turn requires more governance & fraud oversight, both during the purchase and the ownership. It is also more complicated from an accounting standpoint, ( depreciation, write-offs, audits, etc) and complicated finances can be a liability. It's easier if someone else owns & tracks the computers. Once the lease is up, the computer gets taken back to the vendor and replaced with a current model.
Scheduled replacement also makes it harder for end users to sneak in undocumented shadow infrastructure. For example, you don't get situations where that the burned out desktop in the corner with no monitor had been feeding bar code fonts to the mail room printer for the last 10 years, or that there was a piece of ancient CAD software with a special add in from a defunct company that was vital to engineering and the XP laptop it was running on just died. Making everyone hand in the stuff & reinstall every three years keeps many of the worst case shadow infrastructure situations from happening. The software doesn't stick around long once the installation process is forgotten.
And while scheduled replacement definitely increases scheduled outages, scheduled PC replacement almost certainly reduces unscheduled hardware & software outages. It's kind of like how changing all the light bulbs on a rotating schedule, burnt or not, can save overall maintenance costs compared to the costs of assigning one off tickets to change them haphazardly when there are outages, even if it wastes a few light bulbs. Likewise, if you can pick the week your laptop
breaks I mean gets replaced, it should be less traumatic than having it burn out right before the next deploy. And because you swapped computers not that long ago, the process should be better understood.