Yahoo is 5th largest search engine in world and yahoo question answer site has millions of users they want solution for their Problems.
For SEO purpose find some question where you can promote your Website although yahoo provide nofollow backlink but a lot of visitors generated for your websit from yahoo question answer sites and many other benifit in term of SEO.
Answering the any question and creating you own website back link is the part of off page SEO. Answer of the question must be precised so viewer of the answer should like to visit your website. Always chose the question for answering which is belongs to your website and answer the question as correct as much as possible.
Choose a section that you have an excellent command over and do answers most of the questions that fits your skills and you are very sure it will be the best answer because you have exact information about it.
It will give some good traffic to use your website URL along with the answers since the niche is same, people will not mind visiting it and giving you a business.
The link in Yahoo answers are no follow but you can use Yahoo answers to drive traffic to your website. You should include the link in your profile and not in the answers as the moderator might delete your answer if they suspect that you are trying to spam.
Yahoo answers has never been a very important service for generating traffic nor backlinks to a website. Any outgoing link on yahoo answers includes a “nofollow” tag and if someone posts too many links, yahoo’s staff will ban their account. You’d be better off building new links on blogs, websites, or forums that are related to your website.