It's human instinct to belong to a group. People like groups. Some people go to great lengths to get into groups. When people feel like their group is better and more elite than other groups, instinct says hey, I feel secure in this group because it's exclusive and only the best get in here.
This can be applied to private clubs like Club 33 to the little friend clubs we remember from elementary school.
I'm not saying we're like, a primitive society, but this is just basic human nature.