The only reason I have ever turned a quilt is to be able to quilt a border in one pass. I just turn it 1/4 turn so the sides are now the top and bottom. The only reason else I can think is that the machine can only take so large of finished quilt on the take up bar. If the roll of already quilted quilt gets too large to fit in the arm, you would have to turn it around and quilt from the other direction. This is not due to weight, but to the arm depth. Usually on a short or mid arm system and a large quilt.