It has passive legitimacy mechanics. If your prestige is low, your courtiers will dislike you and plot against you and even rebel. Powerful vassals will also disapprove of your requests to change laws. You can be the best king, but you won’t be able to control your children unless they hold land (i.e. switch characters), so if you they grow up to be failsons and faildaughters, once you die your vassals will immediately jump on the wagon to have your new king/queen abdicate or make demands of you with little recourse.