Beacon SupportTwitter analytics only reports on the success of your post on Twitter - how many people may have seen your post (whilst scrolling through their feed), how many liked each post, retweeted etc.
Beacon shows you most of this information along with what impact each post actually has on your website - the real Return On Investment (ROI) - how many visitors you generated from your Twitter posting, what they did on the website (post by post) and how this has improved the value of your website, leading to identified visitors, downloads, orders and requests for information.