A blog is necessary for both SEO and Social Media purposes. As both now come under the tactic of 'Content Marketing' a blog serves a dual purpose. One, it provides content for search engines to index, greatly expanding the opportunity for your website to rank for specific keyword phrases. Two, it offers an interactive element that thrives on social media platforms through commentary, sharing and public opinion. In effect, a blog can be your own PR platform in which to promote your business.