Hey there. I think that the main benefit from Twitter, or one of them, is if when you post a link to your own site, like your blog, for a start you use a shortner like Bitly. If the link becomes very popular and is retweeted by many, it gets listed on various sites which list 'hot stories'. This gives you quality followed backlinks on high page rank sites. Also, the use of bitly to shorten the link allows a super popular link to be stored and give you a really powerful back link from the Bitly site.
Also the retweets within themselves are classed as social signals and are taken into account by search engines, but it is a viral and very popular tweet sent promoting a a url on your own website that gives the most benefit.
Right now our strongest back link is from a blog post that was retweeted over 500 times. The main backlink just shows as a bitly link, with the domain stats of the bitly website.
Hope that helps.
