In developing an HTML5 offline app, a cache manifest is required in order to tell the app which files to cache. If a file changes, the manifest file needs to be changed as well. If a file is added, it needs to be added to the manifest. During the development process this can be quite a PITA.
Enter the dynamic cache manifest. Not only does this handle all the file adds/deletes, but it also handles changed files as it generates an MD5 hash of all files which it stores in the manifest file. So if the hash changes, the manifest changes.