It is for grouping object types. Not 100% sure on what you want to group with but for example you could assign all objects that have a specific instance variable set (e.g. EnemyType:Beast) and put them all in one family called 'Beasts'. Then you can set the turret to target only objects assigned to the 'Beast' family.
I have this object, and during runtime it's Variable, "Type" was set to beast, how do I add that specific one to the Beast Family?