One reason to do it on time is simply to R&R a component that screws into the aluminum head
Porsche for example specifies a 4y for their plugs. Iridium plugs can fire fine for 100+K but having something in the car 7+ years it may be a bit unhappy coming out. I have had basic cars such as a Nissan Rogue also specify time over mileage for plug changes.
For the X1 N55, the service manual does specify 60K for plugs and I plan on doing them this spring when ours hits...60K
- b