It would be extremely useful if electromotive force (EMF) did not vary, but in real life this is only a dream.
EMF in the largest generators is constantly changing, to the point that we often refer to its output as an averaged value, or the RMS value.
Battery voltage is relatively stable, but it is in a continuous state of decay.
Voltage coming out of linear voltage regulators is stable, but it requires stable input power to maintain its constant output. Voltage regulators are also very inefficient and waste power turning it into heat.
As a result, we have to design equipment that can accommodate variations in EMF at the source of power.