I have a Porter Cable about ten years old and a Sears of even earlier vintage both of which use similar systems to set the height of the bit. The base has a spiral groove into which fits three pins on the otherwise smooth body. You screw the body into the base and the extent of the screwing determines the height of the bit. When you reach the desired height you lock the base to the body by turning a thumbscrew which contracts the base. I'm sure everyone has something similar.
The problem is that there's a ring around the base made out of plastic and marked with measurements such as 1/8, 1/4, 3/16, etc but I've never understood the reason for the ring. Logically it would seem to be some sort of micro-adjustment. You get it close to where you want the bit and then by rotating the ring you could move it (say) 1/4 and that would raise or lower the bit by that amount. But the ring just rotates; it doesn't raise or lower anything. In any event you'd be hard pressed to change the position of the body due to the clamping effect of the thumbscrew.
What am I missing here? I can't believe it's purely decorative.