In the context of advisories, it’s simple, to help track documents and avoid confusion. Much the same reason a vulnerability database assigns a unique number to an issue. If there is confusion when discussing a vulnerability, you reference the unique ID and ideally, confusion goes away. That said, why does Hewlett-Packard feel the need to assign multiple tracking IDs to a single document/advisory?
HP-UX running WBEM Services Denial of Service (DoS) http://archives.neohapsis.com/archives/bugtraq/2005-12/0231.html
So this is “SSRT051026 rev. 1”, “Document ID: c00582373”, and HPSBMA02088. Three drastically different tracking numbers for the same document. Fortunately, all three were referenced in the same place this time, but still.. why must vendors do this?