I didn't know I needed one until I ran into the problems with my 308 loaded rounds.
I picked up a case gauge for .223 yesterday, MAN is this thing showing me stuff I have been missing for 20 years......
In one step, I am finding ready to load, needs trimming, and slightly unsized (under sized) brass...time saved compared to just running everything through trimmer. And I am not even considering the saved time from out of spec brass at the firing line.