There are many advantages to using dynamic URLs. As they let your webpages appear as static HTML pages, search engines are more likely to index them (some search engines get jittery at indexing dynamic pages). They're more intuitive and user-friendlier for users. It allows you to put all your content into a database and greatly reduces the number of PHP pages to maintain (I'm a big fan of separating content and code). And crucially, it allows you (or your clients) to edit your website via a web based content management system rather than depending on HTML editors and ftp programs.