When a transformer is in operation, there are a.c. currents in the primary and secondary coils.

The coils have electrical resistance and the currents flowing in them cause heating of the coils. This means that some of the electrical energy is wasted in the form of heat energy.

This loss can be reduced by using low resistance wire for the coils. This is most important for the high current winding (the secondary coil of a step-down transformer - or the primary in a step up one).

Thick copper wire is used for this winding, because wire of a bigger cross sectional area has a lower resistance and wire of a material of low resistivity has a low resistance.

See the factors that affect the resistance of a wire.