The idea of useful work is quite subjective.
Real power is the measure of the rate of transfer or conversion of energy from a source.
Reactive power is the rate of constant circulation of energy between one storage device and another. The flow of energy changes direction every half cycle of the sine wave. The rate of energy continually lost in that process is real power that would typically be converted to heat.
Real power transfers energy that can be converted to some other form of energy like light, sound, heat, or mechanical work. It can also be stored. Real power that represents energy that accomplishes an intended purpose is called "useful power." Real power that does not directly contribute to the useful purpose is said to be a loss, wasted or inefficient.