Let me explain this to you and I'll type really slow so you can understand: someone carrying a gun may enter the store with a firearm partially or semi-partially exposed. Prior to Target's rule, the soccer mom on aisle 5 who saw that or perhaps someone who had a heavy coat on and it's 90 degrees outside, can now go to the manager or cashier and report him.
Before she could do nothing but HOPE the guy is just looking for baby formula and had no ill intent.
And given that all these big box stores have cameras EVERYWHERE, the monitors for these CCTV systems can now alert security if they see someone flash a gun while in the store and avoid a potential disaster, whereas before they would have to wait for that person to start shooting.
It's THEIR rules and their right. Just like they have always been able to stop and check someone for possible shoplifting.