In other words: You don’t need to install any software on your computer (let alone on a web server) and you don’t need to set up an FTP client either. You can register your domain name through the same provider that offers the website builder. And the fact that website builders offer individual support is particularly good for beginners. This means that you don’t need to go and sift through web forums to find solutions to any potential problems.
Just imagine Medium shuts down as did Posterous back in the days (which was a similar service). You will not necessarily lose all your content but all the SEO rankings you ever created. To get it right you need to publish on your own domain name and preferably on your root domain (www.yoursite.com/blog) and not a sub-domain (blog.yoursite.com). This has numerous SEO advantages.