Basically matt nailed if for you, except he used the words "supposedly" - which isn't the case... It's a fact, the same as free floating, or glass bedding, a bolt gun rifle action...
The barrel is left to react the same way each and every time a round is fired. In effect Barrel Harmonics are left undisturbed.
In a non free floated handguard, if you brace the weapon on a bench, or you use a sling, that is adding pressure to the barrel. The barrel then can not react the same way with each shot.
Now, if you have a 16" CQB room clearing weapon, and you are shooting from here to across the street at the further spot, it isn't going to matter much.
If you have a 20" custom built varmint gun and you are shooting out past 300 meters, it can make the difference between a hit and a miss.
CQB weapon, a free floated handguard will help dissipate heat. Which is nice if you are doing a lot of running and gunning.
I generally prefer them all the way around on all my AR's... and most of the police units & military guys we build for are the same way. Though they like to hang a lot of lights and sh!t off their forends...