Because Yahoo Answers is a high-authority Web site, the Q&A pages are indexed by
all kinds of search engines, which means that they'll likely turn up as results for a high volume of searches. If a link to your site is included on one of these pages--and you've provided a good
answer to the asker's question--then you'll be able to snag that highly targeted traffic.
According to Baker, "the SEO effect may not be direct, but the search effect of having those high traffic discussions and links exist is a major plus."