The idea behind a transformer is to supply the power generated by a power plant to the users without much loss of energy. Since the power plants are normally situated for away from the users, it needs to carry the power for several hundred kilometers through transmission lines. But in so doing, lot of energy is wasted in the form of joule heating i2 Rt. So in this situation we use a transformer to step-up the voltage, so that the current decreases, thereby decreasing the joule heating also. And finally we use a step-down transfomer to reduce the voltage and increase the current, so that the power transferred remains the same.