Well, for starters, you can make your pages dynamic - meaning you can have content update for you without you having to manually update it. Like you mentioned, vBulletin is php driven. That's a powerful feature of using php itself. You can store data in a database with php and read it back out to your visitors.
There are many reasons to use php.
Here's a small challenge for you. Make a footer for one of your sites where you use php to update and display the copyright year on each page when a new year starts. It's a small thing, but will help you start learning. Will also give you a valuable use for php. Once you have the php footer on each page, you will no longer need to update any dates on your pages every year.
