Voting has different uses in different contexts. On Meta (here), voting is an implicit, "I agree with your suggestion" or "Oh, this does appear to be a bug". For more on Meta voting, check out the FAQ.
On the Stack Overflow site, as is noted in the responses to this question, it often is used to:
- Given a reward for asking good questions
- To let more interesting and relevant questions rise to the top of filtered lists.
There is also a minor, influence the first impression for a question - I find that when I read a question's title -- if I see several negative votes, I'm less inclined to spend time with it. If I see several positive votes, I tend to make more of a concerted effort to read the question thoroughly. Personally, I spend the most time looking at questions in the active category and while the upvotes may not bump the questions, answers will and I'll be more attracted to questions with higher votes.