Power Spectral Density

What is the physical significance of power spectral density. I know it is used in calculation of bandwidth in digital communication systems, but i could not exactly figure out why PSD? Why cant we just calculate the bandwidth like we do in amplifier circuits (change the frequency and measure the gain)

actually the energy associated with a modulated wave depends upon the the various wavelengths that constitute it.

PSD will give you the average power contribution per wavelength that the modulated wave is possessing.

we cannot calculate the bandwidth as we do in an amplifier circuit because in case of an amplifier circuit, there is a single frequency taken into consideration at a particular instance. The modulated waves that we have in digital systems have a mixture of frequencies and calculus approach is prerequisite to calculation of bandwidth