this is why
in light air, or when going downwind (racing), the halyards need to be loosened. the main gets more curve and thus more power. the jib halyard can be loosened, but is hard to re-tighten when the wind picks up or you head upwind. the back stay on a stiff mast is eased on downwind legs and in light air to affect curvature in the jib luff. for fractional rigs, it is used to de-power the main by reducing draft. also, the outhaul is used for reducing draft on masthead rigs.
in light air, or when going downwind (racing), the halyards need to be loosened. the main gets more curve and thus more power. the jib halyard can be loosened, but is hard to re-tighten when the wind picks up or you head upwind. the back stay on a stiff mast is eased on downwind legs and in light air to affect curvature in the jib luff. for fractional rigs, it is used to de-power the main by reducing draft. also, the outhaul is used for reducing draft on masthead rigs.