The most important use of a cartridge case gage is for setting up 7⁄8"-14 press type full length resizing dies. A common error in reloading is to oversize your brass. Headspace of fired cases can quickly and accurately be measured using this tool and a caliper.
Pistol Max Gages: Used to check all critical dimensions of the pistol case - mouth diameter, base diameter, rim thickness and diameter, case overall length and bullet diameter
Big Book Catalog, Issue:66, Page:513
Sinclair, Issue:4A, Page:035