Is the purpose to compensate for the lack of suspension?
That's my understanding. The kidneys aren't really supported by anything else, and can jiggle quite a lot otherwise. Also I believe they are intended to force a better posture, reducing lower back pain cause by a long time in the saddle. There's a whole debate of course in the modern era as to whether they are good or harmful, somewhat inevitably largely coloured by individuals' subjective opinions being presented as objective fact, as was ever the case on the internet.