Apparent power is the power supplied to the circuit. It is measured in volt-amperes and is the voltage on an AC system multiplied by all the current that flows in it. In a power distribution system, apparent power is what the system needs to be solved. In the real world, loads are rarely just resistive, and most are a combination of resistive and reactive and are either inductive or capacitive in nature.
However, the magnitude of the current and voltage required to supply power to the resistive portion of the load must be sized to overcome that reactance and supply power to the resistive portion. As loads are added, it is necessary to have test equipment available to be able to quantify exactly what the apparent power is, so the electrical infrastructure can be upgraded to handle the extra load.