For more advanced sites that could benefit from things like loops and conditionals etc will need to use PHP files, but if your site is even simplier, then you just need HTML.
StaticPHP takes away the headache of using HTML files with a static site. You still need PHP for running StaticPHP, but you don't need to write any PHP code for your site during development.
Combining StaticPHP's MetaData feature with HTML files gives you the power to develop your site using only HTML files without any PHP, and best of all, no headache that comes with HTML files.