Because managing large sites with only static HTML pages is dangerous for your health.
Because through the dynamic pages, users can interact (Forum, chat)
Because through the dynamic pages we can offer value added services (restricted areas, various services based on user classification...)
Because the information is more easily catalogued.
Because with a few PHP pages we recall a lot of information.
Because keeping the contents up-to-date does not demand particular technical expertise and can be managed by anyone (by Davis Batistes).
It is the simplest way to to pull over a complete portal, thanks to its open source engine, it allows anyone to implement new modules or to modify and to personalize existing modules. (by Micione,
It is very intuitive and easy to learn (by Anonymous)
It is easy to modify by those who intend to personalize the program (By Arus)