Irs in there because, mathematically, rogues require constant advantage (or two weapon fighting and an adjacent ally) to keep up their damage with a fighter (and a fighter who isn't using action surge to boot). Rogue damage is steady. The two chances to land sneak attack, when sneak is the bulk of their damage, is where the class gets it's power from.
Some DMs don't allow stealth to work like it's supposed to for ranged attacks. Not all ranged rogues are sneaky. This offers another solution to get that necessary advantage.