Actually, it is not true that the Emperor's power was only symbolic; the truth is that only people who already were quite powerful could gather the support to be voted Emperor. And they wouldn't have bothered if it didn't mean anything. For one thing, most of the city-states actually owed direct allegiance to the Emperor, and were usually quite happy to support him with money because this direct relationship kept them out of the grasp of the surrounding dukes. Furthermore, the Emperor was crowned by the pope and thus could claim support of God himself, which was quite a big factor in medieval power games.