If you render your content on multiple domains whilst editing your content via a sub domain then when it comes to a cache clear / purge it will get tricky sending all the purge urls to your proxy and CDN services.
The basic Content Expiration module only generates and sends urls based on the path where the content is being edited.
This module allows you to specific a list of domains which should all be purged at the same time.
Example use case
Developed by Ixis.