More so to block on light from the outside of a glass tank or to provide security. Background usually for personal eye appeal.
In this case the Bearded Dragon is seeing his reflection on the glass from the light emitting from the inside of the tank, as glass acts a mirror when light emitted from the inside the greater than the light upside the tank, in order to prevent this mirror effect you need to place a non-reflective object, this case cardboard, in front of the reflective surface, the glass.