Why 3 phase system for electricity distribution?

why electricity distributed as 3 phase system? Why 3 Phase System used in Transmission & Distribution? Most of mains power sockets are single phase, Utilities are in DC But, Power stations generate three phase AC at 120° from each other. Why? The advantage of this is that 3x120° = 360°. In other words, the phases cancel out when connected to a common point.