Most sofa frames, especially solid wood ones, easily outlast the fabric and foam covering them. When a sofa starts looking tired, the frame underneath is often still perfectly sound.
Reupholstery replaces the fabric, foam and sometimes the springs — giving you what feels like a brand-new sofa in a completely different look, at a fraction of the cost of buying new.
It's also a chance to upgrade to a firmer foam or a more durable fabric than the sofa originally came with, tailored to how your family actually uses it today.