Transformer is a static electrical devices which can-
Transfer the electrical power from one circuit to another circuit.
Transfer the electrical power by increasing or decreasing the voltage by decreasing or increasing the current.
Without changing the frequency.
Accomplished this by electro magnetic induction.
In India the maximum power is generated under the 33kv. It may be 3k, 3.3kv, 6.6kv, 11kv and 33kv. Above this rating the genarator size will large which cost is very high. So, it is not possible to use.
But before transmission and after genaration the transformer is used to step up the voltage level. The transformer transmite the power from low level voltage to high level voltage. So, the voltage should be 33kv, 120kv, 220kv, and 400kv.
In the transmission line there have to be resistance and if resistance is present then the losses is also produced. In the other hand we can say that, every conductor has a resistance and that we can not eliminate it 100 persently but we can reduce it.
If we defined the resistance than we should know that the resistance resist the current flow only. So If in the transmission conductor or line current rating is high so loss will also high. For this reason the voltage have to increase and current is decrease through the transformer step up voltage system before transmitting the power to distribution substation. then the power again ragulated and step down to required value to feed to the consumer level.
So in total electrical power system transformer is most essencial devices to feed power from genaration to consumer.