The boom vang controls the height of the boom. As you tighten the vang and lower the boom, you also push the front of the boom into the mast, bending the lower portion of the mast and flattening the bottom of the main sail. The boom vang is used in higher wind ranges where both crew are on the windward side of the boat. As the mainsheet is eased in puffs it keeps the mainsail from becoming too de-powered throwing the sail-trim balance of the boat off. Downwind the vang also keeps the top of the sail from spilling off too much.
This control line is also often led to either the CB trunk side or even the side tanks for adjustment during hiking. The cascading set-up below creates an 8:1 purchase and provides 2 controls for adjustment from either side of the boat. For those that desire more purchase, make the H225 at the mast a double (H226) and move the H225 to the mid-vang point, creating a 12:1 purchase (and requiring >46” of line).