The boom vang keeps boat wakes from upsetting the boom and shaking the shape out of the sail. It also keeps the boom  down when broad reaching and running. When the reef is in, the boom vang tension has a lot of control over the  fullness/flatness of the sail.  I also added a second bail and shackle 65" up the gaff from the throat for the peak halyard block when the sail is reefed. If  the peak halyard block remains in its original position, 42" up from the throat, when the reef is in, there's a lot of twist in  the peak of the sail. This twist may be desirable in very high wind, but at the lower end of the reefed sail range, you loose  some power if the peak halyard block remains at the original 42" position. With the outhauls, luff tension, boom vang, and  peak block position, you have a lot of control over the sail shape. I prefer to reef early, but keep the reefed sail full and  powerful at the lower end of the reef range of wind speed. It is at this point that tightening the boom vang will flatten the  draft considerably and move it aft. If you're sailing beyond that, consider dropping the gaff, moving the peak block back  down to allow maximum twist off, and tighten the peak outhaul some more.  Here's a picture of the peak halyard block on one of it's two bails. There is a shackle swedged to each bail with SS wire.  The block is fastened to the shackle with a button lock fast pin, which is attached to the block with a small lanyard.  Moving the block takes just seconds. A shackle is ready at both positions, and the fast pin is always at hand.  69