There is no standard mains voltage throughout the world and also the frequency, i.e. the number of times the current changes direction per second, is not everywhere the same. Moreover, plug shapes, plug holes, plug sizes and sockets are also different in many countries. Those seemingly unimportant differences, however, have some unpleasant consequences.
Most appliances bought overseas simply cannot be connected to the wall outlets at home. There are only two ways to solve this problem: you just cut off the original plug and replace it with the one that is standard in your country, or you buy an unhandy and ugly adapter.
While it is easy to buy a plug adapter or a new "local" plug for your "foreign" appliances, in many cases this only solves half the problem, because it doesn't help with the possible voltage disparity. A 120-volt electrical appliance designed for use in North America or Japan will provide a nice fireworks display - complete with sparks and smoke - if plugged into a European socket.
It goes without saying that the lack of a single voltage, frequency and globally standardised plugs entail many extra costs for manufacturers and increase the burden on the environment.