Why you use 33kv for power transmission?

Power loss during power transmission is predominantly due to the current drawn, which heats the wires. Power dissipated is current x voltage and voltage is current x resistance, so in any wire with fixed resistance the power loss is proportional to the square of the current flowing. The higher the voltage you use to transmit the power down the distribution network the lower is the current, hence power is distributed at the highest voltage that is practical, then the voltage is dropped to the domestic level at the destination sub-station or by a transformer on the power line pole. Major distribution systems operate at over 100 kV, with regional ones at lower voltage.

33 kV (not 'kv'!) is the primary distribution voltage, not a transmission voltage, used in the UK. Other distribution voltages are 66 kV (not very common) and 11 kV. Transmission systems operate at 400 kV and 275 kV.