why is the concept of average power useful?

Average power gives us the measure of work done per unit time. Average power is mathematically expressed as,
 $\overline{P}=\frac{\Delta W}{\Delta t}$
This average power is not an instantaneous power rather it is the average power which is important in such a condition when the average power of the entire process within a given time is calculated. For example, average speed of a moving car is different from its instantaneous speed at a particular moment.