It will tell you if your brass and loaded rounds fall within the maximum specs for the round.
The only time I have had a problem with reloaded 5.56 rounds chambering was when I made the mistake of trying to seat a few bullets a bit deeper after they were already seated and crimped at a longer OAL. When I did that the bullet did not push further into the case - the case bulged at the shoulder and the rounds would not chamber. If I had a case gage like the one above, I would have known immediately. I have a gage now and check every one of my reload rounds.