Hmm, that is an interesting question. I have rarely heard of either a rod or a staff as being objects of comfort. They are usually used as tools of punishment.

Maybe they represent strength, seeing as the person who usually holds both is usually responsible for taking care of the people below him.

Sort of representative of that thingy that kings hold. (so sorry folks, I have been working in French for the last four days and my brain is not working in English tonight.)