Speaking of mechanics...No one mentioned the duchy alignment...
Now why would it matter? You may have noticed that if you are the lord or knight of a region, your civil work, police work, and bureaucratic work is more effective. Now what if you and your knights are all occupied and can't tend to your region? Worse, your region happens to be a recent acquisition from a week before, and currently hates your realm with seething passion. Well, keeping it an Imperial region would not be a very good idea for most cases. You would lose out on the slightly more effective work that anyone in the duchy can do. That is not that much of a big deal, but depending on the situation, your realm size, and external factors, you might want as much optimization as you can get.
Aligning to a duchy also means that your region will go along with the city during secession. As well, in places that have the option of appointing lords to regions, the duke then has appointment rights over any region in his duchy. And finally, a new character can start in any region contained within the duchy of the realm he chooses.
