From Ars Technica, I saw a link to Brad Delong’s notes on a talk with Google founder Larry Page.
I’ll use the same quote that Ars used, because it’s the interesting one:
Larry Page: "It wasn't that we intended to build a search engine. We built a ranking system to deal with annotations. We wanted to annotate the web--build a system so that after you'd viewed a page you could click and see what smart comments other people had about it. But how do you decide who gets to annotate Yahoo? We needed to figure out how to choose which annotations people should look at, which meant that we needed to figure out which other sites contained comments we should classify as authoritative. Hence PageRank. "Only later did we realize that PageRank was much more useful for search than for annotation..."