It'd be nice to offer to relinquish your lands in a kingdom in exchange for others. If you were say, Duke of Toulouse, and you conquered Jerusalem and wanted to make yourself King of Jerusalem then (and I'm going by what you said, since I haven't tried this) you can't, you have to be independent, which means the Kingdom of Jerusalem would come with large holdings in southern France? No, better to have a chat with the King and say "look, I'm going to become king of Jerusalem so I'm going to pass all my holdings in your kingdom to my cousin who would swear fealty to you, I hope we can still be friends x."
Incidentally I was playing as France yesterday and I conquered my way into northern Spain but realised I didn't really want the land or trouble, so I wanted to make the King of Aragon title then give it to my 3rd son, so the dynasty continues, however I'd already given him a small plot of land in France, I would like to say to him "give me back that land and I'll make you King of Aragon", so I wouldn't have the issue of ugly borders leading to war later.