Question

Why voltage level for transmission is very high?

Answer 1

It is a cost/efficiency issue.

Since  P=VI where P= power, V= voltage and I = current

P=VI, the higher the voltage, the lower the current for a same amount of power transmitted.

Any transmission line has a resistance, and the losses due to the heating of the line are given by

 P_loss=I² R

Thus, the lower the current, the lower the losses in the transmission line.