It's a couple of things:
* The QB being able to see the pass rush and go through his progressions faster. With all the exotic blitz and coverage packages, an extra split second can make a huge difference.
* RPO's. Keeping the defense guessing until the last second whether it's a handoff, QB run, or a pass. There's so much more versatility out of the gun, and a mobile QB is far more dangerous.
I will say that seeing teams consistently in the gun on 3rd or 4th and less than a yard is infuriating. By the time a slow developing run play gets going, the defense is already in the backfield.
It's the same way I feel about screen plays and bubble screens in the red zone. With the field that compressed, the spacing necessary on those plays is very limited and the defense is coming downhill. Just absurd, yet you constantly see it.