Why are high voltages used when supplying electricity over long distances?
A few thoughts...
A higher voltage will probably make electricity cheaper to deliver.
Suppose you have an appliance that requires a certain wattage in order to operate. If the voltage is double, then you only need half the current. If you have half the current, then the supply cables will suffer half the voltage drop for the same resistance and therefore lose half the power in the form of waste heat.
In order to reduce the loss you either need to reduce the resistance of the supply cables - requiring more expensive cables or a shorter distance to the substation - or increase the voltage.
If I remember correctly, in the UK, the voltage used for long distances conveyed over large pylons is about