The dark moulded base under the gold plinth is doing something specific: it puts a shadow where the eye expects one.
A large piece of furniture standing directly on the floor reads as heavy, because there is no visual break between the object and the ground. The traditional cure is a plinth - but a plinth in the same colour as the piece simply extends the mass downward.
So the base here is finished dark while the carved plinth above it is gilded. From a standing position the dark band reads as shadow rather than as timber, and the gilded carving above appears to hover clear of the floor. The sofa keeps all of its presence and loses the visual weight at its base.
The height of that dark band is calculated from the seat height and the likely viewing distance - too thin and the effect disappears, too deep and the sofa looks as though it is standing on a step. Our artisans set it out on the drawing and check it against a full-size elevation before any timber is cut.