1 - multiple material zones in props and clothes.

With props ( especially scene props ) having multiple material zones allows you to adjust detail in textures/shaders for close-up or far away shots more easily.

With clothes, multiple material zones allow you to design your own color/texture scheme by just using shaders and tiles... increases a model's use-ability.

2 - If you're selling a bedroom or big set... it helps if the walls, ceiling and floor can all be hidden separately so the camera can move around at any focal setting.