PHP can be a boon to any website designer who knows how to use it correctly. Small amounts of PHP have become common places in most professional websites, particularly large ones with slick designs that require regular reloads of tables and other navigational tools. In a regular website created without PHP, each individual page will require the coding to create the menus being used. Although there are ways around this such as using a template or certain CSS features, the real benefit of PHP is how it transmits this data.
Unlike other web code, PHP is a server side language. This means that most of the work is done by the servers rather than the users home computer, meaning reduced load times on the user's side. On the whole, this is a huge benefit. The one draw back to this server side model is that pages with large amounts of PHP on them are often resource hogs and will require a significantly heartier server than other similarly busy, PHP free websites.
If you're looking to make a site that requires a large amount of reusable elements, PHP might be the way to go. If you're working commercially, your IT team will probably already know whether your servers are PHP capable and if not they'll have plenty of information on how to upgrade them. If you're working on a smaller, personal site most web hosting companies will have PHP server deals. Be sure to aim a bit high in your spec requirements, though. PHP tends to be more of a resource hog than you might initially anticipate.
The dynamic content could be anything from a simple GIF image to an entirely customized web page rich with features ranging from basic HTML all the way to graphically intensive interactive Flash. There are other more creative uses as well, but it'd be virtually impossible to name them all given the remarkable flexibility of PHP.