Suppose you want to cut a tenon of a specific diameter, and you want to do it accurately and repeatedly, say on a bunch of near-finished shafts (for ferrules, rings, or...). And further assume that you can't just set your tool and cut that tenon in once swack. If you're good, and your crank handle dials are good, you may be able to get pretty close.
Now imagine you have a DRO on your lathe. You put your first piece in and you take a rough cut, followed by a "finish" cut - all oversize from what you really want. Without moving your cross slide you zero out your X-axis readout. Back off your bit, move the tool away form your work (Z-axis), and then crank back in on your cross slide until you hit zero again.
Now measure your practice tenon, and input that value for your X-axis. After that you can cut your tenon, and all remaining tenons, to exactly the diameter you want - time after time, with exacting precision. and you're not bending over, squinting at your crank dial, trying to sneak up an a relatively coarse alignment point. Instead, you're looking at a nice bright digital readout screen, watching the tiny increments go by.
That, my friend, is an invaluable use for a DRO - and it's just one example of how much a readout can save you time and improve your results.
TW
