The motor is an inductive load with a well-defined power factor (known to manufacturers). So, it is straightforward to understand the KW specification of the motor.  

Also motor converts electrical energy into mechanical energy, so KW (real power) makes more sense. The output power can be easily calculated if the motor’s efficiency is known for a specific load.

$$\text{Output power}=\text{Efficiency}\times{}\text{Input power}$$

So, the KW rating gives a fair idea about the mechanical load (coupled to the shaft) it can drive.