This page describes the issues which embedding license metadata attempts to resolve and basic steps for implementation.
When a person creates a new piece of media (eg: a new photo) and they wish to release this creation under a Creative Commons license they must indicate that somehow. The usual method is to put a bit of text next to the image on a webpage saying something similar to "This work licensed under a Creative Commons Attribution 3.0 license" with a link to the license. There are common examples of this on many web media hosting services such as flickr or vimeo.
However, as soon as someone else downloads the file from the webpage there is no longer any contextual license information. It is now up to the downstream user to be accurate and faithful to the terms of the license. This undoubtedly means knowing which license the work is released under and unless they made a note of it somewhere else they are apt to forget.
If, however, the license information was embedded into the file at the time of creation (or sometime soon after by the creator) no matter where the file went (webpages, desktops, CDs, etc) the license information will always be readable by a downstream user.