I visit SO and related sites each morning, and often each evening as well. In between, I often bump into them when I'm just doing my regular work. I need to know how do something, I head to a search engine, and often the results include SO-etc questions. If the answer helps me with the problem I'm facing, I give the OP an upvote. By asking the question before I even had the problem, this person made it so the answers are here waiting for me. I'm grateful for that, so I give an upvote. As well, any answers that look useful get an upvote.
On my reglar visits it's different. Questions get upvotes if they make me think "good question", or if I answer them. Answers get upvotes for being right, downvotes for being wrong (or too short or a thousand things.) And questions get downvotes if I can't edit them into being better, or can't just vote to close them. All the other sites routinely nag me to "vote on questions too" because under normal circumstances, I rarely do.