These units are morphed from the Mutalisks.The Guardians have a very strong anti ground weapon which ahsa a great damage rate and also has very high attack range.The refresh time is also good.On the other side the Guardians are costly and slow organisms and also its place in the Tech tree is far from the first ages of the Swarm this makes the morphation of the Guardians difficult.But at some multiplayer games the production of the Guardian can be a necessity.Especially if your enemy defends a little island with anti air buildings the usage of the Guardians will be very very efficient.Here using the guardians you must protect them from air units because these organisms can not attack air.