Lets take this example. There two boys name Mark. We can use their last name to differentiate them from each other.
Also they come from different houses. There are other members in the house. The individuals in the house can address each other simply by their first name. For example, father addresses Mark simply as Mark. However if the father wants to refer Mark in the other house, he would use the full name.
Also someone outside the house would also refer to the full name when they want to refer boys or anybody in the house.