It is complicated and analog and digital have different reasons for the standards. Basically it to keep the luminance and chroma components of a signal under control so as not to cause problems in transmission or display of the final picture. An analogy might be recording an audio signal on a recorder at the wrong level, like putting a line level signal into a mic level input. All kinds of distortion will occur. That's simplified but it's basically to assure that the signal coming out is as close to the signal going in as possible.