Why is there alternating current in my wall socket?
From Wiki:


  Transmission loss
  
  The advantage of AC for distributing power over a distance is due to
  the ease of changing voltages using a transformer. Available electric
  power is the product of current × voltage at the load. For a given
  amount of power, a low voltage requires a higher current and a higher
  voltage requires a lower current. Since metal conducting wires have an
  almost fixed electrical resistance, some power will be wasted as heat
  in the wires. This power loss is given by Joule's first law and is
  proportional to the square of the current. Thus, if the overall
  transmitted power is the same, and given the constraints of practical
  conductor sizes, high-current, low-voltage transmissions will suffer a
  much greater power loss than low-current, high-voltage ones. This
  holds whether DC or AC is used.
  
  Converting DC power from one voltage to another requires a large
  spinning rotary converter or motor-generator set, which was difficult,
  expensive, inefficient, and required maintenance, whereas with AC the
  voltage can be changed with simple and efficient transformers that
  have no moving parts and require very little maintenance. This was the
  key to the success of the AC system. Modern transmission grids
  regularly use AC voltages up to 765,000 volts.