Maybe it's because we want to make sure we have a solid backup if we run out of lip gloss. Or to guarantee we always have a dance partner to deter greasy gropers. Or someone to tell us our hair looks really shiny and we don't look fat. Or perhaps it's simply because we need to be surrounded by a feminine security blanket while functioning in public.
I don't have the definitive answer, but this hilarious "Huge Group of Girls" video mocks the cat-pack phenomenon, and it is pretty damn spot on. Watch, and educate yourselves on the ways of the bar-hopping woman.