Great question! This has been a hot topic, and with the SEOmoz bi-annual report that came out recently and showed a very high correlation between social shares and high rankings, its even more important.
The answer is pretty simple. The more visibility your content gets, the greater the chances of people linking to it. So social media helps SEO by spreading the word and giving exposure to your content and brand. And this increases the liklihood of someone linking to you.
After all, if they can't find you they can't link to you.
I'm open for consulting calls if you would like to elaborate on the subject.