Sometimes you have only a small amount of fabric to use or you are unsure if the fabric will work with the décor you want. In order to save time and materials, I make color copies of the fabric to test out in scenes before I cut the real deal.
Paper is excellent for curtain tests. You can quickly try out different fabrics to pick the right one for the space. Or, if you have a fabric that would never hold its shape or is too thick for miniature draperies, you could use the paper copy itself instead of fabric. You would just need to have more care when curling and pleating the paper into shape.
You can test pillows for scale and color.
Upholstery is trickier, but you can get a rough idea here as well. :]
