The backing is the tell
On a felt panel the gap between slats is fabric. Up close the eye reads texture where it expects shadow, and in raking light the felt goes grey rather than dark. Our backing is painted MDF or the same veneer as the slats, so the gap reads as depth.
There is a practical side too. Felt is a textile bonded to nothing structural, so fixings have to find a batten behind it. A rigid backing takes a screw anywhere: a shelf, a bracket, a television.