The primary and secondary winding can be constructed as a step-up or step-down transformer to match the load in the electrical system
Preventing the equipment from getting spikes and harmonics from the mains
Safety. There will be no conductive connection between the live part of the circuit and earth. With the isolating transformer, there is no danger in touching the live while the body is earthed.
By connecting the electrical system safety ground to the neutral conductor on the transformer secondary, it eliminates neutral-to-ground voltage and noise. This resolves reliability problems for microprocessor-based electronics.