Welcome to the Metadata Millennium: A Complete Overview of What Metadata Can Do for Publishers
There are whole categories of metadata that we simply don't have room to cover here but I will mention one more metadata standard: METS, the Metadata Encoding and Transmission Standard from the Library of Congress. METS is a model for encoding descriptive, administrative, and structural metadata regarding objects within a digital library, expressed using XML. We have already discussed descriptive metadata (such as subject classifications, keywords, etc.). Administrative metadata keeps track of things like when a document was created or revised, and by whom and for what purpose; it also includes such things as source metadata and rights metadata. File metadata records the file types, file sizes, etc. for a resource. Structural metadata records not only the structure of the given document, but can record how it fits into some larger structure. Link metadata records all the resources that are linked to by the document or publication. Behavioral metadata records things like interactivity and other scripted features, which with EPUB 3 are becoming more commonplace.
Finally, one essential purpose of metadata is to make publications accessible to everyone. Though people usually think this means making visual aspects accessible to the blind, that's only a small part. The same things needed to make content accessible to the blind are also needed for others: people with low vision, people with dyslexia, and people who are unable to use the gestures common on tablets are a few examples.