Helmut has it, as usual!
Any equipment which uses measuring devices/indicators etc. to demonstrate parameters which are necessary for process control (e.g. heat, pressure, time, speed etc.) have to be of known accuracy etc. Therefore, they should be calibrated/verified.
Any equipment used subsequently to inspect/test particular features/characteristics (e.g. size, shape, finish etc.) after further processing and are used to determine product conformance also have to be of known accuracy etc. - and therefore calibrated/verified.
It's simple - really..........!!