PHP can retrieve information from a SQL database and list it as posts and threads. HTML doesn't.
HTML could retrieve information through Javascript, but it's not as secure as PHP and can cause problems for some internet browsers. It also takes longer in some cases.
Right now, I'm using a setting where 40 posts are viewed per page. If I were to use HTML, that would be a HUGE file to load up. If I were to use PHP, it would be a small simple-coded file that reads elsewhere and brings it all up in HTML format.
