It has huge set of functions to make your SEO easy. The plugin is so popular because of its ease of use, flexibility and effectiveness. I got the super result on domain aquotes.org.
Configuring and customizing the plugin is very easy and user friendly. Starting from basic meta tag information to advanced settings can be done under the same roof.
Plugin additionally provides the APIs so that other themes and plugin can extend the functionality. It has support for the custom post types and Google Analytics as well. It can also generate META tags automatically. If you are beginner then you won't even need to look at the options because it has wonderful default settings which suits almost everyone's needs.
If you are advance user and wanted to customize plugin, then without touching the messy PHP code you can customize almost everything using a very user friend GUI.
In short, the plugin stands for its name and provide all the functions of SEO in one package.