Answer 1

html is a markup language, which is in its escence static. Imagine you want to have a clock in your page. using html only, you as the programmer would have to manually change the time inside your html program periodically.

php allows you to create dynamic content, since it is a scripting language. Another benefit of php is that, since it was made specifically for web programming, it allows you to retrieve and share information inside the web.

Any page that allows you to fill forms, to login or to store and retrieve data, or simply displays or manage dynamic information, is probably using php to achieve this.