You get a bigger demesne and vassal limit too.I went to check on an old save and didn't have the option "press all claims".
I already can create an empire. I don't know if I should. What are the advantages vs disadvantages? I already have 3 kingdoms, can usurp one and create even more. I know I could get more prestige but don't want my vassals getting too powerful or starting to desire my Kingdoms, nor making my succession too bothersome since I have gavelkind. I can only see a reason to create the Empire: I'll need the CB to conquer Aragon.