According to the description on the All Site Content page in SharePoint,
Items in this list contain HTML or text content which can be inserted into web pages. If an item has automatic update selected, the content will be inserted into web pages as a read-only reference, and the content will update if the item is changed. If the item does not have automatic update selected, the content will be inserted as a copy in the web page, and the content will not update if the item is changed.I see several good uses for this right out of the box. This feature existed in SharePoint 2007, and like nearly every content management feature in SharePoint, it is not new nor is it exclusive to SharePoint. Back in the day, FrontPage had a feature that let you store content in one place and insert content tokens throughout your site. When you published, FrontPage would replace every instance of the content token with the centrally edited bit of content. And many other site management tools did this. Even so, I'm glad I discovered it in SharePoint 2010!
I'm currently researching a couple of issues related to using Reusable Content. First, in my client's environment, this list does not seem to be available to subsites, but having it available to these would be super helpful. Second, I want to be able to put tokens (i.e., SharePoint controls) in their master pages so that content managers can manage content like the site's slogan and copyright through the SharePoint interface.
When I have discovered the solutions to these issues, I will post them here.