There is a survival advantage in conforming. Over the course of evolution, those groups which have succeeded through co-operation in their established practices are the successful ones. Thus the urge to conform and copy others' behaviour is one which confers an evolutionary advantage. Of course, we don't simply conform to such practical behaviours as gathering food, we also conform to religions, fashions, fads, extremist political movements and so on. To a large extent, human society is not a society of individuals, as the society of leopards, or even lions, is - albeit the individual lions are lumped together in groups.
Human society is composed of groups, superorganisms.
THe cohesiveness of groups that conformity achieves is a valuable weapon in a world where groups must act together to compete with other groups. That the decision may be arbitrary is less important than that it is unanimous. Humankind, I suggest, has always fragmented into hostile and competitive tribes and those that found a way of drumming cultural conformity into the skulls of their members tended to do better than those that did not. When we examine the results of experiments in social psychology, it is worth asking ourselves what, if anything, we consider to have been proven and within what intellectual and social framework the 'proof' has been delivered. Since Thomas Kuhn's The Structure of Scientific Revolutions , it has been commonly accepted that science does not proceed on the basis of the patient observation of facts, from which general conclusions are then drawn, but, rather, that science operates within the framework of a dominant 'paradigm'.