Since each part of a web page is separate, it is much simpler to update something across many pages, all at once.
For example, if you have a change to your website’s navigation you only have to change it in one place (the file that holds navigation information) and it is updated on all pages that contain that file.
If you had a static website, you would have to edit every single page in order to make a change to the site’s navigation. This is tedious, repetitive, and prone to errors.
Because a dynamic site pulls together a bunch of page bits to make a whole page, you can store your content (and other parts of your site) in a database.
The advantage of this is that you can easily access and edit your content in a variety of ways or even have it load across multiple websites (for example, if you had two distinct brands with some overlapping content.)
Databases also make it easy to search your content, categorize it, load it in dynamic ways (such as displaying the 5 most recent entries about sock puppets), and back it up. All of this is nearly impossible with a static site.
Dynamic websites with a content management system (CMS) make it simple for a non-technical person to create and update the content of the site. Because the various parts of the page are all separate, a content creator won’t need to know any HTML in order to create a new page or article for the site.
You can simply concentrate on writing the content and the dynamic site takes care of putting that content into the right spot on the website. With a static site the content creator would have to know HTML or employ the help of someone who does in order to create a new web page for a site.