"Why do people want their black tanks to be clean inside?"
It prevents build up because the contents will dry out and turn hard. Then build up again, and again, and again. Over time, the tank will be full of noting but solids and rock hard built up 1/100th of an inch at a time (or more).
A simple method to keep the black tank cleaned out and spare the usage of water, if both your black and gray outlets exit at the same spot, is to get a Flush King style (3rd) slide valve with a clear section. Start by draining the black completely, regardless of how full it is or is not until noting more comes out. Then close the 3rd Flush King valve and open the gray tank and let it back-flush into the black tank. Wait 10 seconds and close off the gray. Then open the Flush King valve (3rd valve) again and a BUNCH more will come flushing out of the black tank. Repeat 3 times, and finally rinse the hose with the remainder in the gray tank. The back flush method saves water, cleans the black tank, and pulls MUCH more out of the black tank that would have been destined to just sit there and dry out!