Share buttons allow your target audience to promote your content for you. Each Like/Tweet/post/e-mail/+1 etc. sends a signal to the search engines that your content is valuable and useful. If your blog is on the same domain as your corporate site, this is a great way to build your site's overall value. Internal linking allows pages of your site to share link juice with other pages, so heavily linked blog posts pass their trust factor along to the rest of your site.