Reduced hydrodynamic resistance and thus more speed or less required power. But only properly designed catamarans have this advantage. This might not work for heavy boats overloaded with superstructures and furniture.
Reduced vertical accelerations (slamming) on the seaway. From our measurements during sea trials, catamarans possess 30-50% lower accelerations compared to similar monohulls. This means more comfortable riders on cats or catamarans can maintain higher speed on waves.
High initial stability, higher freeboard and flotation, duplicate propulsion units, etc. Catamarans possess very high survivability, say, in case of collision. They are extremely safe in case of passenger crowding, fast turns, etc.
Plenty of space with excellent accommodations on deck/saloon/flybridge area. Say, for 24′ catamaran yacht, the usable area of the accommodation is doubled, compared to a monohull. Another example: up to 70-80% of newly built high-speed passenger craft worldwide are catamarans – why? Because of ride comfort, space, safety.