You have to remember that people were not appointed to positions in the middle ages: families were. Neither appointing a good warrior nor a good manager was any guarantee that their grandson would be as good.
However, if you appoint someone who commands the loyalty of thousands of men through feudal/clan ties, you have a pretty good chance that their grandson will command the same loyalty.
Half truth. SOME titles were hereditary, by no means all. Wise rulers always ensured they kept a good portion of the realm as non-hereditary grants to ensure they had lands for use as gifts, rewards, bribes what have you.
