People want quick answers, not browsing documentation. SO is useful in the sense that it provides answers to specific questions which a google search finds instantly.
I love reading docs too, so I can get a deeper understanding. But sometimes I just need an answer in a hurry.
Or they don't want to understand the issue at hand, so they put in a hack, when obtaining a complete understanding of the issue which might take 5 minutes of their time reading the docs, would provide a better solution.