Why gateways?

Why does the HTTP/1.1 spec need to talk about gateways?  As a spec of protocol,
not server implementation, it goes without saying that a server implementation
can call on other servers via other protocols.  I note that the entry for
"gateway" in the index (of rev-02) lists only one occurrence: the entry for
"gateway" in the glossary.  (There is at least one more use of "gateway" in the
spec: on page 12, where gateways are allowed in the chain of intermediaries.
Do you really want to say as much as 1.4 says about the ways that these other
protocols can work?)