The former is the whole point of implementing backports support; the
latter is needed to get a net-retriever version that knows about the
extra Packages file to be used at runtime.
Warning: That doesn't ensure packages are actually installed from there,
as net-retriever hasn't been backported yet.
Status: for stretch-backports branch only.
Signed-off-by: Cyril Brulebois <email@example.com>