First, search engines are changing their valuation of sites that have a significant amount of likes/tweets/plus-ones/etc. From the perspective of the search engine, these validations are actually far more relevant/valuable than "backlinks" (which is an older way of increasing your SEO rankings by getting other Web sites to post links back to you).
Second, to ensure that you are capturing as much of this Social SEO as possible, you need to make your Web site "social optimized" so that your visitors can easily share it. This includes embedding share buttons and ensuring that your meta tags include the right descriptions/photos for sharing (i.e. so that the right images and words appear in the description of your page when people share it on Facebook). This way, you can capture as much Social SEO as possible.
Third, it is important to encourage visitors (as well as your team) to share your site often. Yes, this will help with the SEO rankings, but more importantly it will expose your brand to the friends/audiences of those who share it. In this way, you can transform customers into salespeople for you.
All that said ... if your only purpose in pursuing this is to get higher placement in Google, forget about it. Search ranking is an only an outcome. The deeper purpose should be to focus on building relationships with customers through which they are invited to be a part of your brand (in this case, by sharing your site with their friends/followers). If you focus on that, the rest will follow.
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.
Quite simply, social SEO is important because it allows social properties to rank for relevant key phrases in the search engines.
Having social media accounts rank for a key phrase, or even better - a brand, can help to accomplish a few things:
1) Increase brand awareness
2) Aid in online reputation management
3) Positively influence click through rate of other search results
Outside of these benefits, social SEO helps to actually make social profiles and pages more interesting and useful to users.