I want one!
It's for boring holes to size without all the faff of stopping the machine, adjusting the screw, tightening down, rinse repeat until out to size. Also, as someone above said, ideal for cutting internal circlip/o-ring grooves, with an appropriately shaped cutter.
Each "click" will move the head out a defined amount (how much depends on your device, some are adjustable). So to make, say, a 20mm hole (pre-drilled 15mm) when your rachet moves 0.1mm per click, you'd simply bore to depth using the quill drive - or by hand... - raise the head, grab the control ring, count (say) 5 clicks, release, bore, raise, grab, click-click-click-click-click, bore, 3 more times (if my mental arithmetic is right) and presto - a 20mm hole, in a fraction of the time it'd take with a regular boring head. Now imagine you have a dozen of such holes to make...
You could also use it for roughing out a taper, either way up (but if it's narrow-end-up, don't try to withdraw the tool all the way!); so plunge Zmm at diameter X, raise, click, plunge z-1mm, raise, click, z-2mm, etc. or whatever your measurements are. Also ideal for cutting a hole where a bearing needs to go; endless1 uses!
It's kind of pointless for "accessible surface" facing, as you'd just use the biggest end/shell mill you're comfortable with & do X or Y passes to taste; but good to give you a nice flat bottom on a blind hole you've just bored or drilled... so long as you're good at counting the clicks!
1Almost