See what Google says:
Fast and Secure: A managed WordPress host offers the latest technologies to make sure your site is fast. They also take care of the security of your WordPress site so that it's safe from hackers and malware. Core Updates: They take care of the core WordPress updates and thus, helps you to secure your site.
If you are not an expert or you have limited time to read and maintain your WP site, then is the best solution. I remember once I have a Joomla 1.5 managed site and they did all thee work for me. But some people want to manage in order to learn and to have an activity like hobby.