Kind of piggybacking off Wake's post. Reasons for longer rail/handguard which covers more barrel:
1) Less deflection when shooting off cover/barricade;
2) More room to stretch out your support arm for faster transitioning and comfort; most guys don't shoot that whippy grabbing the mag well or the carbine length handguards, longer is optimal;
3) Less chance of burning yourself on the barrel when shooting from suboptimal positions;
4) Aesthetics - even without the above advantages, I prefer the longer rails.