Host header with an empty value?

Quoting http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-22#section-5.4

   A client MUST send a Host header field in all HTTP/1.1 request
   messages.

   If the target URI includes an authority component, then
   the Host field-value MUST be identical to that authority component
   after excluding any userinfo (Section 2.7.1).

   If the authority
   component is missing or undefined for the target URI, then the Host
   header field MUST be sent with an empty field-value.

Can someone elaborate on the last sentence and give an example? Thanks.

Zhong Yu