It's simple. For every minute a developer spends writing an answer, many people read the answer. So if the best answers are voted to the top, and there is little duplication, the site has saved a lot of programmers time.
SO is a wonderful site, I remember the days before SO when I used to get annoyed by sites like Experts Exchange that required a paid membership. Though, I agree SO has saved a lot of time with a great source for common questions and best answers, great answers. But, I also have to say that for many developers, specially in their early days, it provides a lazy alternative to learning.
A lazy debugging practice is to find a question that generally has a subject line almost exactly the same as the error message and pick the first answer, without even trying to understand it. I see the difference personally when I worked on .Net/Java and when I was working on a applet for Gnome. The latter is not very popular and so I have to resort to documentation, IRC etc. I might spend more time initially but I end up reading the documentation, develop a better understanding and even create my own snippets which in turn save me time eventually. To summarize, SO saves time but one might tend to leech on to it instead of learning. I guess some metric of how many times a person revisits the same question might be a good indicator of this.
Not to mention how many times I have seen a comment like, is this homework; show me what you tried, instead of an answer.
What's wrong about the "is this homework" comment? Why would you (this is a generic "you") expect someone else to answer your question _for free_ if you haven't bothered to actually try solving it yourself? Where is the courtesy and etiquette in that?
I think SO/SE's aggressive discouragement of leechers and high question-closing rate makes their website better for everyone. Except the leechers and "help vampires", that is.
