So, if you don't pin down the priority of deb-multimedia, virtually every
audio- and video-related package on your system will be replaced with the
deb-multimedia version, which for the sake of stability is very likely a
bad idea.
So it is safer to lower the priority of deb-multimedia and that of
deb-multimedia backports even a bit more, so that the official debian
packages remain the default, deb-multimedia packages the first
alternative if you actually want/need a newer version or a version with
some extra features and deb-multimedia backports the last choice if you
really, really need that version.