If you're really interested in an answer, this may help:
828451 - Why service packs, updates, and security patches may require the
Office XP CD-ROM:
http://support.microsoft.com/default.aspx?scid=kb;en-us;828451
Part of it is that the patches are kept small by being only incremental
binary patches.So Windows Installer needs to see the original file that was
used for your installation in order to do the delta.
In some cases, full file patches have been made available that mitigate the
need for the original source file.
There are other ways to mitigate the need, by how you install the software.
Additionally, Office 2003 has created a local installation source on the
hard drive that contains source files so that the installer doesn't need the
CD.
--
Susan Ramlet
Office MVP
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