The contrast from bookstore to single site Q&A is exaggerated, but I was around before Stack Overflow. The problem was not just that there wasn't a single (popular, highly active) site, and not just that you had to sift through search results. It's that if you didn't find your answer, you didn't know for sure where to post your problem, and actually expect to get answers. You might post it on a personal blog or forum, and then wait a very long time to see if a response would appear. Often it would not (or it was something you already tried or something else that didn't work), and you'd be on your own, and if other people were lucky, you'd document the eventual solution (but often, you did not.)

Stack Overflow gave you a central place for you to ask questions, and (within a pretty reasonable time frame) start to get suggested solutions to try and consider.