When creating a Drupal site for clients, many people use a temporary hostname for development purposes. What's more, that hostname is not generally on the same domain as the production site. I'm sure I'm not the only one to use client,mydomain.com to develop the www.client.com site.
When you — like me — use Drupal in a multisite setup, where the same codebase is shared between many sites, you'll have per-site settings, files and images. These are located in a directory structure that's based on the hostname you're accessing the site under. Eg: the files and settings for client.mydomain.com live in the sites/client.mydomain.com/ directory.
However, when the time comes to deploy the site to its production hostname, those settings and files need to live somewhere else. Moving them can result in link and image breakage on the front-end, which is never a good look and means you might need to painstakingly update links and images on all your pages.