Power transformer ratings use kVA or MVA because they depend on apparent power, which represents the combination of voltage and current regardless of load power factor. Losses and heating relate to voltage and current, not only to real power in kW or MW. Using MVA allows the same transformer to serve different types of loads with different power factors as long as current and voltage limits are respected. It also simplifies coordination across systems, since other apparatus like generators and switchgear are often rated in apparent power too.