Search engines love social signals. Social signals (the appearance of your website on social media platforms like Google+, Facebook, and Twitter) gives your website "authority." Much like how regular pages linking to your site informs search engines that you have content worth reading, social media mentions tell the search engines that people are talking about you, and that you are relevant.
Having a social media account is crucial to your SEO campaign. It also should be active. Engaging with the public online not only brings you visitors (via links shared on your pages and your fans' pages), it keeps your name relevant to current conversations.
It is also a great marketing tool in your overall marketing arsenal. Email marketing can be a great tool, but spam filtering engines can be very aggressive, often filtering emails that your customers have signed up to receive into the spam folder. Unlike email, current social media platforms do not send your comments or tweets into the dead zone. Social media readers engage more often with businesses that they are actively following.
Social media technology also integrates well with your website. You are able to easily add "like" and "+1" buttons to your pages to have your visitors promote your page to their own friend networks. Plugins to popular content management systems (such as WordPress, blogger, Joomla, etc) allow you to automatically post new content from your website directly to your social media accounts, allowing you to instantly spread the word that you have something worth reading.
Local business owners know that there's no service quite like that from a local company. Do your research when selecting someone to do your SEO. Don't fall for a page where Baltimore is just one of their "regions" only to find out they're not even in Maryland.