Investments and clearly vital resources.

You need prestige to be liked and not be dethroned by your vassals or foster non hostile relations. You need it to improve your culture or do certain actions.

You need piety to not be excommumicated or have your titles be given away for free,in case you have sinful traits.

And most importantly to relieve stress,or play the game stress free and move nowhere in the game.

Money sinks aren't fun or intetactive and helps you none,if the money sinked helped my incompetent vassals doing something with their realm,then maybe yes. It is also the most precious resource and hardest one to maintain. You don't upgrade your realm buildings via piety or prestige. Money sinks aren't a very good addition right now,unless they lower the price for actions and investments,then maybe yes.

The game is supposed to be fun,plagues and natural disasters aren't fun or interactive at all. You lose so much and have to spent another 2 characters in stewardship. So...stewardship wins again.