Longer shaft provides more distance potential you can create with the same clubhead speed with less effort or more speed with the same effort. You potentially create a bigger swing arc, which can hit the ball farther. The longer club could slow down a fast swing, which could be beneficial. For an older person, or possibly one who is heavy, it might help him create a fuller pivot in an effort to let the longer club do its thing, which is to swing longer.
 

A average golfer first few tee hits with a lighter and longer driver will experience that his/her shot go farther with that same effort and swing tempo, a 15 to 25 meters distance gain is not uncommon. Already the longer shaft swing arc assisted more clubhead speed, the lighter weight will even allow the arm and hand to swing faster to gain additional speed. Golfer with swing style and tee shot of 150 to 180 meters will find that such club very beneficial for distance.

While your best hit may go farther, the total of all your hits may not be greater because the longer shaft is harder to control. The longer the lever system gets, the more difficult it is to control. You can only swing a club at the rate of speed that the strength of your hands and arms can bear. The problems with a longer lever system is that if you don't have good hand-eye co-ordination and can't time the swing, you can't move the club any faster at impact, so the ball won't go any farther. And it may not go as straighter with off-center hit.
 

The long shaft can accentuate swing problems. With the longer shaft you are going to be farther from the ball and swing on a shallower, flatter plane, more around the body. You might become more consistent with the longer driver and consequently your iron play might be affected. If you have a problem controlling your weight from the right the left foot movement during your swing, it is going to be compounded with the longer club. The longer clubs is harder to use for the golfers who has a late release because the clubhead is farther back when it's time to hit the ball.
 

The effect of hitting a much lighter shafted club with the same loft angle is that the ball trajectory get higher. For the best tee the ball must fly to the optimum trajectory for maximum carry distance, too high ball flight may resulted distance loss. For strong players a longer shaft and too light a club can be more difficult to control, he tee shot may hit farther but not any straighter.