Why would you answer a question that isn't worth your upvote?
You can vote as you wish but it's most helpful if questions and answers that are useful and well written are voted up, that brings them to the attention of everyone ahead of Q&A's with fewer votes.
Questions that are useful to a very limited number of people, maybe only a couple of people (the asker and answerer), don't need to come up ahead of others.
Answers that took a lot of effort to research or that answer a difficult question ought to receive some reward. Answers that are incomplete, don't answer the question, or seem snarky, rude, or belittling shouldn't receive upvotes over better answers that might be forthcoming.
Someone might have spare time to answer a question that benefits a very few people or might be interested in storing their answer somewhere since they've had the problem too (and want a reminder). Despite their own interest they don't feel it warrants everyone's attention.
It's like asking: You've read the question and didn't answer, "If a question is worth reading, why isn't it worth voting on?". The effectiveness of these sites runs on voting as well as great questions and answers, if you don't have a question to ask or see one you can answer then voting is another way you can help.
From the Help Center:
Help Center > Privileges > vote up
What is voting up?
Voting up is how the community indicates which questions and answers are most useful and appropriate.
When should I vote up?
Whenever you encounter a question, answer or comment that you feel is especially useful, vote it up!
You have a limited number of votes per day, so use them wisely.
Follow that link for more information and to find out: "What happens when I vote up?".
Help Center > Privileges > vote down
What is voting down?
Voting down, also known as "casting downvotes", is how the community indicates which questions and answers are least useful.
When should I vote down?
Use your downvotes whenever you encounter an egregiously sloppy, no-effort-expended post, or an answer that is clearly and perhaps dangerously incorrect.
You have a limited number of votes per day, and answer down-votes cost you a tiny bit of reputation on top of that; use them wisely.
Follow that link for more information and to find out: "What happens when I vote down?".
Help Center > Asking
What should I do when someone answers my question?
Decide if the answer is helpful, and then...
Vote on it (if you have earned the appropriate voting privilege). Vote up answers that are helpful and well-researched, and vote down answers that are not. Other users will also vote on answers to your question.
Accept it. As the asker, you have a special privilege: you may accept the answer that you believe is the best solution to your problem.
To accept an answer:
- Choose one answer that you believe is the best solution to your problem.
- To mark an answer as accepted, click on the check mark beside the answer to toggle it from greyed out to filled in.
- You may change which answer is accepted, or simply un-accept the answer, at any time.
Accepting an answer is not mandatory; do not feel compelled to accept the first answer you receive. Wait until you receive an answer that answers your question well.
Ideally everyone does the right thing and everything works as was expected, in practice it doesn't always work exactly that way; this is why we rely on the efforts of multiple people to answer but don't allow duplicate questions, similarly multiple votes tend to average out to the community consensus as to what is better (sometimes things that are clearly wrong but thought to be funny or an amusing take on something will also receive a lot of votes).