Actually it just appears that many countries use the 240V as the standard voltage. In reality, there
are subtle differences in the standard voltages used in line and also the frequencies in which the power is transmitted. For example most of the European countries use 230V, 50Mhz as the standard. Some of the African countries use 220, 50MHz as their standard. So eventhough the differences in voltages are not much, they still require different types of sockets. Also the sockets vary based upon the connection types, grounding preferences of a country. The result - LOT of varieties of sockets.
You can get a detailed info here if you are interested.
http://en.wikipedia.org/wiki/List_of...nd_frequencies
Regards,
Prash