Operators need a way to hide outdated images from users, while at the same time images should still be usable.

From the proposed spec:

"Cloud operators supply *public images* that can be used by end users to boot
servers.  An example is an image containing the CentOS 7 operating system.
Such images must be updated as security concerns, etc., are addressed.  In
Glance, however, image data is immutable, so each update results in a new
public image.  Further, operators do not want to delete the "old" public
images, as end users may require them for different use cases like server
rebuilds.  As a result, the default image-list for end users becomes very
large.  Further, the default image-list may contain multiple CentOS 7 images,
for example, making it difficult for end users to determine which image to
use."