Why to use kva?

The kVA (kilovolt ampere) is the vector sum of real + reactive power in an AC circuit. The kW (kilowatt) is a measure of the real power in that circuit. Inherently, a circuit will not require only real power, but also reactive power. Thus kVA is a more meaningful value when considering sizing equipment (such as Transformers, bus work, breakers, etc.) because this equipment must be sized for the total current drawn, not just the real power usage.