8.12 TRACE and entity

As I implement TRACE, I'm puzzled by this passage:

                                          A TRACE request MUST NOT include an
      entity body and MUST include a Content-Length header field with a value
      of zero (0).

Why must there be a Content-Length: 0 header?

1) Wouldn't the absence of a Content-Length header be equivalent?

2) If a server does not find a Content-Length header in the
request, is it obliged to return an error?

Dave Kristol