What is a fundamental purpose of a step-up or step-down transformer?

Supply a neutral reference on the secondary side that is different than what is supplied on the primary side.

Supply a ground reference on the secondary side that is different than what is supplied to the primary side.

Deliver a voltage reference on the secondary side of the transformer different than what is supplied to the primary side.

Deliver the same voltage reference on the secondary side of the transformer equal to that supplied on the primary side.