You don’t, but you can use them on the command line (MDI) via a G90 or G91 command.
Example: You are at X287, and you want to move to X127
MDI G0 X127
Now let’s say you wanted to move just 1.0mm (Positive)
MDI G91 G0 X1.0
I mostly use it when probing with the Additive Aerospace probe when locating to the center of a circle.
G91 G38.1 X10 F88 (then I add a - in front of the 10 to probe in the minus direction, and then change the X to a Y…etc. It can be a time saver.
Yes I could just reset my XY zero in (about) the center of the hole I am probing, but I want to know the exact position from X0Y0.
I hope this helps.