I vote in every election, including primaries, for a few reasons:

1) I care what happens in my city and state, and the people we are voting for are going to have a large impact on our daily lives, from transportation to taxes to public safety.
2) Many of our local elections are close races, especially when it comes to governor, because the west side of my state is liberal and the east side of my state is conservative, so I want to do my part to ensure good policies.
3) I want our elected officials to feel accountable. If they know only 20% of the voters actually voted, I think it gives them the sense that no one is paying attention, and no one really cares. I want them to know we are paying attention, we will weigh in, and they had better pay attention to what we want, or they can kiss their careers goodbye.
4) I don't feel its ethical to turn my back on taking responsibility for my community. If things go wrong, and I didn't vote to prevent that, then I'm partially to blame.

In addition to voting, I think it's a citizen's responsibility to pay attention to what elected officials (and, vicariously, appointed officials) are doing, and what they're planning or advocating for. When something isn't right, people need to speak up, talk to their legislators, and do something about it.