A back wall provides a way for the infrared light to return back to the scanner. The infrared light hits the back wall and bounces back. This provides a sharper image and will provide more accurate and precise measurements.
If there is no back wall, the image will not be as sharp and you'll notice a bit of "fuzziness" in the scan or artifacts, particularly between the thighs. The measurements will also be larger than expected.
The wall should be 2-5 feet away from the rotating platform (55-150cm).