Having been there and done that, I can tell you exactly why. It's because the border fabric and the quilt edge tend to stretch as you sew. If you have not pre-measured and pre-cut, you will be highly likely to end up stretching the edge and sewing more border fabric to the edge of the quilt than the edge of the quilt should have. This results in ruffles at the edge of the quilt top. If you don't notice this until after you have quilted, you have a quilt with ruffley edges that will never lie straight and flat.