What is the point of duchies if you're king?
Example: I'm king of France and count of all counties inside Normandy. I'm also the duke of Normandy.
What benefits does this 'duke of Normandy' title give to me exactly? I tried to give the 'duke of Normandy' title to another noble, and it didn't seem to change anything except he was now the duke of a duchy without any county (the counties still 'belonged' to me and seemed to have been ejected from the duchy).
Sorry if this is not very clear, I'm very confused.
- It gives you prestige
- Makes it easier to handle your kingdom (controlling 60 count vassals is harder than controlling 20 duke vassals)
- Allows your vassals to expand on their own (i.e. if your duke owns 2 of 3 counties in a duchy, you can press the claim on your own, or you can let him do it himself.
- Dukes are more likely to make improvements on their own (since they gain more money), so less micromanagement.
The main reason to me is #2 above. My first game I didn't make any duchies, all counties reported directly to me, which was all fine until I wanted to change a succession law. One of the requisites is that no vassals can have a negative opinion of you. Try to achieve this when you have 60+ counts to take care of and a bad king with no heirs. I just couldn't do it.
As for your second issue, when you give the duke title to someone, it's only a title at that point, you should either give the duke the counties in the duchy if you own them, or if the counties are controlled by your vassals, do 'transfer vassalage' of the other counties in the duchy (barons most likely) to the new duke if you want to remove the 'desires such and such county' penalty.
Also... there will be situations where you can grant a duchy to a person with a claim on another duchy in order to keep that duchy after you press the claim on it. If you bring a guy to your court with the intention of pressing his claim on a duchy, I'm pretty sure he will become independent once the claim is pressed and won if 1) the duchy is not in your de jure kingdom, and 2) He does not already have a duchy in your kingdom. Same goes for counts: if you want the guy with the claim to become your vassal after you grab the county for him, he needs to already be a count under your rule (you can avoid this by having him be a member of your dynasty, or also if the county being taken is within your de jure kingdom.
In other words... you can strategically give out duchy titles to people in your court with duchy claims in order to ensure that a successfully pressed duchy claim yields you a duchy (rather than making a new, independent duke who thinks you're his best bud but isn't part of your realm).
Prestige! You get a certain amount of prestige for each title you hold. (I'm not sure how much off hand.) The only caveat is when you're king you can only hold two duchies without incurring a relationship penalty with your vassals.
Edit: Also if you give a duchy to a noble he will then desire all the de jure counties under that duchy. (giving you a relationship penalty with him unless he owns them)
This question is a bit old but I have a different take on it. If you own all counties in a duchy then whether you want to keep that duchy around depends on how many duchy titles you currently own. If three or more, then the duchy is causing you a negative opinion modifier from all your vassals.
In this case destroy the duchy title, don't give it to another character. If you own all counties and give the duchy title to another character, they will get a huge negative opinion modifier unless you give them all of the counties!
If you own two duchies total or less, then keep the duchy until this changes. It's giving you some prestige as has been said, whereas destroying it costs prestige.
I also want to add that this applies only in case that you own all the counties in the relevant duchy. If you don't you will incur -50 opinion with all de jure vassals of the duchy. Same goes for kingdom titles, which can usually most conveniently be disposed of immediately after a crusade.