.When sites grow, you might have realized that across multiple pages lots of code repetition occurs. This is a problem when you need to change certain parts of a page, that affects many or all pages.
.Furthermore, its hard to introduce new developers to code someone else has written. It takes a long time to get familar with the code.
.Enterprise class (big) web applications need structure, one HTML file per page with PHP code is not optimal => things get confusing, hard to work in teams, etc.