Apparent power is what the wires and windings actually experience: volts times amps (times √3 for three-phase), measured in kVA. It exceeds the real power doing work whenever the power factor is below one, through S = P/PF — a 10 kW load at PF 0.8 obliges the supply to carry 12.5 kVA. Transformers and generators are rated in kVA precisely because their limits are current and voltage, not the usefulness of the power passing through.
A detail that quick mental math often misses: converting a kVA rating to amps involves no power factor at all, because the rating already is √3 × volts × amps. A 100 kVA transformer at 480 V delivers 120 A of full-load current at any load PF; the PF only decides how much of the kVA is real power. That is why nameplate currents match the I = kVA × 1000/(√3 × V) figure exactly.
Sizing equipment runs entirely in this currency. A transformer study converts the load to kVA — from current via √3·V·I/1000, or from kW via kW/PF — applies a growth factor for spare capacity, and picks the next standard ANSI/IEEE C57.12 rating: a 480 V, 100 A load is 83.1 kVA, becomes 103.9 kVA with 25% growth, and lands on a 112.5 kVA unit at 73.9% loading.