This is for a boom vang but because it is useless on catamarans it no longer comes on new boats (14s or 16s) as on the older boats. Many people just drill them off to get it out of the way.
Boom vangs are primarily used on monohulls usually to control the leech of the mainsail downwind or upwind when the wind is strong enough that you need to spill it significantly to stay flat in a gust.
If you watch your leech as you dump the mainsheet you can see the upper part of the leech twisting off which makes you loose a lot of air and slows you down alot. A vang keeps it from twisting off. The alternative to this on catamarans is the traveler. Downwind the traveler should be let out and the main not let out excessivly to keep the leech tight and not twisting off. That is the same reason many sailors play the traveler upwind in heavy air instead of the mainsheet to keep the leech constant and not twisting off.
-Todd