I want rpg book to be complete and self supported. Rules are IMO less important than setting, so I expect rpg books to contain less rules than than lore.
I don't want to use calculus to estimate damage, and realistically a big table of bonuses/maluses for an action is useless in real life as the GM has to set up a difficulty without breaking the game flow. But I want to know about that lady who rules her noble clan by poisoning her opponent, that cult of Eunuch monks who can see the future and the new religions who spread and is about to kick out the new God. These are the elements I need to create story.
Rules are an element to make the setting alive, and sometimes a whole part of that setting, for example D&D alignment and mage who forget their spell are a huge setting choice