While on a broad reach, the staysail is eased and the clew hooks back towards the staysail tracks on the deck. At the same time, the leech twists and spills the air out of the top of the sail. This all leads to decreased efficiency in the sail and less speed through the water.
By simply rigging a barber hauler, all of these problems can be corrected.
The barber hauler pulls the clew out towards the rail, and removes twist from the leech of the sail. Now the sail can be properly set to use the air on a broad reach in the most efficient way possible.
When the winds are light, this added efficiency means you can continue moving at a reasonable pace instead of seeing the iron genoa! In our case, we were moving at 2.5kn under main and staysail. This may seem way too slow for most, which is what motivated us to rig the barber hauler. Once the barber hauler was set up, we began moving at 3.5kn. Still slow by most standards, but we were still moving under just sail with out the use of a motor.
When it is blowing harder, we don't set up the barber hauler because we are moving fast enough and we don't feel the need to improve the sails efficiency.