Because mini-split systems only cool the specific areas you desire, ductless air conditioning is a very energy-efficient home cooling method. Cooling is focused on desired areas and cool air is never sent to unoccupied rooms (unless you leave the unit on!)
Since cool air is cooled as it passes over the AC unit, there is no loss of cooling as air travels through ducts around your home. This means your AC can work less to reach your desired temperature.
Mini-split systems used for heating are also more energy-efficient. Instead of creating heat with fossil fuels, mini-splits transfer heat to your indoor air.