I had learned that if one does not have a walking foot -
that the bottom layer - against the feed dogs - will feed the fabric a bit faster than the top layer that is against the presser foot.
So that one can more easily ease in fullness if that's the layer that is against the feed dogs.
Stretch and Sew classes had a motto - "Sew with a relaxed bottom" - meaning that the top was stretched to fit the layer being eased in -