Her house has high ceilings, so she needed a medium weight fabric. In addition, the living room she made the curtains for is on the street side and there is a street light only a few meters from the window. This meant that the curtains had to be reasonably thick to keep the light out. Velvet works well for this, but she added a backing just to be sure.
Adding the backing was a lot of extra work, but I think it was worth it. I saw the curtains up without the backing and they definitely hung better once the backing had been added. In addition, the backing will protect the material from the damaging effects of sunlight.
The finished curtains were beautiful and certainly much nicer than the readymade ones we saw. When I need new curtains, I know who I will be asking to make them for me.