It would be for a quick change. I don't have the micrometer inserts, I could have one for .223, one for .308, one for 45acp and 1 for 9mm. All I have to do is label them, when I change calibers, just change inserts and powder and go, no adjusting. Same goes for the calibrated micrometer but like you said, its easy enough to read it and record the numbers. I didn't realize it would be that accurate.
Jon