It doesn't necessarily have to exist for its own sake. Plenty of furniture is considered art, and is exhibited by art museums, but that furniture was designed to be used.
If you mean, was _primarily_ designed to be used, then I don't think that is necessarily the case. For example, certain pieces of furniture are sometimes designed by an architect to complement their overall design. Of course, they want the furniture to be used just as they want the building to be used, but for many I think the art comes first.
