Why Is A Transformer Rated In KVA?

Actually the answer is quite simple: KVA is a Transformers safe Current rating for the Primary & Secondary voltages applied. Sample: 5KVA = 5K (5x1000) or 5000 Volt Amps. Volt Amps is a NEMA rating evaluating the (%) of Voltage Drop at the applied voltage specified for the transformer. The KVA rating will "safely" supply the specified voltage applied for your application.
5KVA at 480 Volts AC Primary - 120VAC Secondary is rated good for approximately 40 amps.
The Transformer will be pulling Approximately 10.5 amps at 480VAC - full load.

Per Unit measurements are essentially percentages of the rated capacity. For instance, a power transformer might be rated at 10 kVA, and have a rated loss
If a transformer has lower than rated voltage applied, the KVA is reduced by the same ... Transformer Rating (KVA) Average Sound Levels (Decibels
Just as horsepower ratings designate the power capacity of an electric motor, a transformer's KVA rating indicates its maximum power output capacity