Why does official.xml exist at all? Does anyone remember?

the git log explains why ;)

commit a287f5b0b4481bd10761ae0cbddcff6bef02f646
Author: David James <davidjames@chromium.org>
Date:   Wed May 8 15:32:47 2013 -0700

    Add official.xml as superset of all other manifests.
    
    Currently, the sonic builders try to build sonic.xml, but this does not
    work particularly well for paladin or release builders since we have a
    master/slave build system. If each slave were to need a separate
    manifest we would need to sync each manifest separately and somehow keep
    them in sync, which would be difficult.
    
    Creating a single manifest to rule them all makes things easier. This
    official manifest contains some repositories that only bots have access
    to. We still create board-specific manifests for developers who only,
    for example, have access to a single board and don't have access to the
    all official board manifests.