No medieval king became Emperor with no reason. Maud became Empress because she was related to a Holy Roman Emperor, and she was probably very vane. Alfonso of León was crowned Emperor of All Spain, but he didn't became king of everything else in Spain. He was just king of León for all real effects. Prestige stuff.
I've said before that I'm against Emperors popping out everywhere. There must be a good reason to become Emperor, because an Imperial crown is serious stuff, it can grant you considerable spiritual power, especially if you claim it to come from Roman Emperors, along with all their cesaropapism. The Pope will have something to say for sure.