When fabric is rolled on a bolt, it often goes on crooked. You may have noticed this if you cut strips from your fabric and they form a “V” instead of being nice and straight. Due to the extra width, and being folded an extra time, wide backs are more susceptible to this.
Here is a wide back I received recently. When it’s unrolled it from the bolt, it seems fine, doesn’t it?
But look what happens when I align it so the selvages are even:
That’s almost a 4 inch difference!! By the time the backing was squared up, there might not be enough left for your longarm quilter to load it on the frame.
Tearing allows for straight edges on both sides of the yardage, and those edges will be at a 90 degree angle to the selvages. A square back means I can quilt your top without pleats or puckers on the back – something that will make both of us happy!