Adjustable parallels are very useful tools. In cutting key ways as es I mentioned, for example; Tapered keys are often used on large machines such as mechanical presses to retain and index gears and flywheels to shafts such as crankshafts. These keys are fitted very closely usually with meticulous hand filing to final fit.
The key way on the shaft is often cut in a boring mill with an end mill cutter. To measure to a good average size, the adjustable parallel is placed in the key way and expanded. One end is left out of the slot and measured across with a micrometer. Likewise, the mating keyway in the gear hub is often cut with a shaper/like slotter machine. The parallels also facilitate an excellent means to measure. The depth in the hub is tapered to 1/8 inch per foot and the gin headed key is cut with a matching taper. The key is driven in and removed with graphite powder and the indicated high points filed down. The process is repeated until the key is at the desired depth with at least 80% coverage all around. When properly fitted these keys hold parts in place tenaciously under very severe shock loads. The fitting can require many hours of filing in between driving a very snug key in and out with a five pound short handled hammer.
Heavy work yet very well fitted. Starting out with accurately sized keyways is crucial to getting the job right. The right tools matter.

Fixture building and machining setups have near endless applications for adjustable parallels as well.