Ed,

Back when I was working every day in CNC screw machine shops, they were used with custom tips to measure shoulder heights on parts, depths of holes, flatness, put a cylindrical part with a hole in it on a pin to check concentricity, all kinds of things. When making hundreds or thousands of the same part on a screw machine, they can be set up for a tolerance band, and if the part dimension being checked fell out of the set range, the background color changes from green to orange/red color.

They are a little bulky for use on anything but the most robust Noga magnetic base, but with that you can use them for dialing in a part in a 4-jaw chuck in the lathe because you can set the sweep range of the digital electronic "needle". Very cool tool. Oh yeah, I forgot to mention that they don't have an internal battery and as such need to remain tethered to the DC power supply. Not exactly a good thing around a spindle.