From RFC2616:
This method (HEAD) can be used for obtaining
metainformation about the entity
implied by the request without
transferring the entity-body itself.
This method is often used for testing
hypertext links for validity,
accessibility, and recent
modification.
The reason why HEAD is preferred to GET is due to the absence of the message body in the response making it using in scenarios where you want to determine if the content has changed at all – a change in the last modified time or content length usually signifies this.
Also, a HEAD request will provide some information about the server setup (whether it is IIS/Apache etc.), unless the server was masked; of course, this is available in all responses, but HEAD is preferred especially when you don’t know the size of the response. HEAD is also the easiest way to determine if a site is up or down; again the irrelevance of the message body makes HEAD the ideal candidate.
I’m not sure about this, but RSS/ATOM feed readers would use HEAD over GET to ascertain if the contents of the feed have changed.