IE, it resolves a urn:isbn: url to an amazon / barnes & noble / etc style url.
Applications of this:
* Ecommerce; identifying products by EAN/UPC barcode if it exists
* Geo coordinates in URLs to place names (on wikipedia, perhaps)
... it normalises URIs that it knows about, which mean that you can post http://www.hubmed.org/display.cgi?uids=16099373 or http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=16099373&dopt=Abstract or info:pmid/16099373 or even http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6SYV-4FX23FY-3&_coverDate=08%2F31%2F2005&_alid=356223062&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=4844&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=0af86d8bdaa4e097bc8ac51186461586 and they'll all be identified as the same object (the first three will be linked by PMID and all four will be linked by DOI). The same applies to books, which are linked by their ISBN.