For as long as there’ve been in-process gaging systems, they’ve used hydraulic or pneumatic actuators to drive them. Both are simple to implement, well-understood, and since most machine tools come equipped with a hydraulic pump, a compressed air connection, and all of the hoses, piping, filters, manifolds, and valves needed to make these systems function, tapping into them for gaging purposes has made good sense.
Or at least, it used to. Piggybacking a measuring unit onto a machine’s hydraulic system means a bigger pump and motor are needed, increasing investment while driving up operating costs. Compressing hydraulic oil causes heat, and heat causes thermal growth in machine tools, never a good thing when precision machining is involved. Hydraulic oil gets dirty, can potentially leak, and must be disposed of, raising concerns about the environment.