I don't think that's silly at all. It's healthy to have a "What's in it for me?" understanding of something since we're all driven by our motivations (intrinsically and extrinsically) whether or not we acknowledge them.

Witnesses need votes in order to perform their job as a witness. The top 19 create new blocks every round while the others are rotated in based on the weight of support they get. We all get to control who holds those important positions. If, for example, someone was running a node which missed blocks or if their actions weren't in the best interests of the community, the users can (and should) remove their support. In essence, by voting for trustworthy witnesses, you get to ensure the Steemit network you enjoy continues running smoothly which protects your investment.