Because power = volts x amps. If you double the voltage, you must halve the amps otherwise the power would double also.

Because if you push harder, you do more work, even if you end up pushing for the same distance.  Pushing a Range Rover 100 yards is more work than pushing a Fiat 500 the same distance.

Or another analogy is flow and pressure - if you have a litre/minute of compressed air at say 100psi you can probably get twice as much work out of it as if it were to 50psi.

In early electrical writing (and even the odd bit of legislation) voltage is described as "pressure".

   - Andy.

If you had a tank of water 110 metres high and the flow down the pipe is a certain amount (say 10 Ebee units).

Another tank of water at 240 metres high and the flow is 4 and a bit Ebee units, then they are both working the same.

V (Electrical Pressure) x Amps (current flow) = Watts (power).

If you lift a weight of 10 pounds upwards one foot, that is ten foot pounds of work.

If you lift a weight of 1 pound up ten feet then that is still ten foot pounds.

Hope that helps.

To possibly add to your confusion with another analogy, if you consider an electrical circuit as being a system of carrying buckets of energy round the circuit to get that energy from one place (the source) to another (the load).

How fast the energy is being moved overall is the power (in Watts).  The current is how many buckets per second go round the circuit, and the voltage is how big the buckets are.  The total power is therefore what you get when you multiply how many buckets per second go round, by how big the buckets are - Power = current x voltage.

Therefore if your buckets are 2 and a bit times bigger (240V vs 110V) you only need to move them a bit less than half as fast to get the same resulting power.

The maths/physics bit follows - so stop reading now if you prefer:

Power (in Watts) = Joules (energy) per second; Voltage = Joules per Coulomb (Coulomb is a bucket in this analogy); Current = Coulombs per second.

Power = Voltage x current.  Power = (Joules per Coulomb) x (Coulombs per second) = Joules per second. 

Thanks for your replys think I got it now Happy Christmas guys best wishes from me

But Top Tip, don't try and do it in the real world i.e. your American washing machine won't do a cycle in half the time in the UK, it will just go bang and let the genie out!
Other things, more volts needs more insulation, more current needs more wire.