A photo box is very useful for getting a nice photo of your catch while keeping it in water. It can still "breathe" and won't flip out of your hand while you ready your camera. Additionally, identifying some species is much easier when their fins are spread - which they are in water and aren't in your palm.
You will notice things that you'd never see if the fish was in your hand, like the metallic glow in the stripe of a duskystripe shiner or around the eyes of a creek chub. You know that a creek chub has a dark dot at the back of it's dorsal fin, but if the fin is collapsed, as it is in your hand, you'll never see it. You might not see the fish's "moustache" either.