1.Harmonics Isolation:
In a practical world, many equipment have harmonics. Now these harmonics will affect the performs of electrical devices sometimes causing fire. These harmonics are caused by non-linear loads. To reduce the harmonics in the power system, a simple 1:1 transformer is used. The transformer acts as the connecting bridge between the mains (supply) and the equipment. The harmonics caused by the equipment will be isolated in the secondary circuit. Thus the supply is not affected by the harmonics.

2.Impedance Matching:
According to the "Maximum power transfer theorem", maximum power is transfer only when the impedance at the source is equal to the impedance at the load.( In simple terms, Impedance is to AC what Resistance is to DC). By using a transformer between the load and the supply, their impedance can be matched and maximum power will be transferred.