git-update-index -q

The git-update-index man page entry for -q says:

       -q
           Quiet. If --refresh finds that the index needs an
           update, the default behavior is to error out. This
           option makes git-update-index continue anyway.

Can someone explain this to me?  As far as I can tell,
git-update-index without -q does not error out after the first
update.