One word..
WOW!
Or rather..Drooool!
This supplement is chock full of rules and ideas on the building and running of empires. It provides details on the economics of empires, the effects of both religion and magic on empires, how empires wage war, and how empires (and kingdoms) can come into being and/or fail and break apart.
It also expands upon the roles of the nobility, providing details on the differences between the levels of nobility in term of fief incomes, the roles and responsibilities of nobility to both their subjects and those above them, and the effects of their political standing within a realm and upon the "mood" of the populace.
Additionally, it includes rules for the construction and maintenance of various buildings ranging from shrines and smithies to castles and cathedrals and incorporates the effects of weather and various seasonal events on realms.
Empires not only provides background for any RQ campaign, its rules fit seamlessly with the character-driven RQ rules, allowing a GM to run a campaign at the metagame level while providing a more immersive environment for players. It gives a GM more than enough info to bring the players into the arena of high-level politics as the players increase in power and influence. With Empires, RQ players can truly become nobles and even heads of state and experience the benefits, and headaches, that come with taking on such lofty positions.
I've been playing RQ in its various iterations for decades (literally) and this is one of the most useful RQ supplements I've seen in all that time.