Since each part of a web page is different, it is much easier to update something at once on multiple pages.
For example, if you want to make a change in your website navigation, you will need to change it in only one place on the changing website. Once you have changed the file containing navigation information, it will be updated on all pages containing that file.
At that point, if you had a website standing and wanted to update navigation, you would have to edit each page to make changes. This can be a tedious and repetitive process, prone to human error.