What is the need of extra high voltage transmission lines?

it is cheaper to use high voltage of transmission because, it is cheaper to boost the voltage

up really high and keeps the current low, also the big pylons with huge insulators will

reduce the energy wasted. the transformer have to step the voltage up for efficient

transmissions and it bring back down to safe usable levels. the voltage is increase

by a step up transformer it's then reduced again at the consumer end using a step

down transformer.

The only thing missing from the above narrative is the reason itself:

Every conductor has some resistance. When an electric current flows through a

conductor, the resistance of the conductor causes loss of some of the energy. The

loss is LESS when the voltage is higher and the current is smaller. The big pylons,

the huge insulators, and all the step-up and step-down Transformers wouldn't be

necessary if the voltage was the same 110 volts all the way from the generating

plant to your house. But shipping it at high voltage saves more energy than the

cost of all that extra infrastructure.