Why DOCTYPE Declarations for XHTML?

In the Working Drafts published recently for various flavors of XHTML 1.x,
the section on conformance requirements seems to insist on a doctype
declaration with (boilerplate?) text that looks like this:

: There must be a DOCTYPE declaration in the document prior to the root
: element.  If present, the public identifier included in the DOCTYPE
: declaration must reference the DTD found in [some appendix] using its
: Formal Public Identifier.

Could someone from the WG - or anyone with similarly privileged access to
discussions held in camera - explain this requirement?


Arjun