As implemented in Mach 3, G31 is the command for use with - well anything, since, although it is " a straight probe" you can with a little bit of thought use it for all sorts of position finding.
The basic command line is "G31 X1". With that the X axis will move towards position 1 at a low speed. If a pin designated as "straight probe" (on Config/Ports and Pins) is earthed in the time the axis is moving, it will immediatly stop. The command can be applied to any axis with the same result.
On my machine I have a two pin din socket on my control box - providing an earth lead and a connection to the "probe" pin. I have a lead with a two pin din plug at one end. The other end, the earth wire is fastened to a dog clip and the "pin" wire is soldered to a piece of copper clad board.
If I want to use G31, then I plug in the lead. and clip the dog clip - usually onto the tool or drill - the copper clad is then placed as a target. The thickness of the copper clad is known, so when the axis stops I know it's position and can adjust the DRO's accordingly. In actual fact, my machine is earthed in any case, and I can do away with the earth clip.
If your question is should you have it or not, I would suggest it is one of the most useful and "quick" methods of position finding for any machine.
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