If you try to sew lots of small horizontal fabric pieces together, you are going to find it hard to get a straight strip to use. You either end up with a curving strip that won't lay flat or one that veers left or right. The foundation keeps it going straight and you trim the edges back to the paper edge. You sew with a small stitch length so that the paper is easy to tear off AFTER you sew the strips in a quilt top. You tear them after sewing them in so that you don't pull out any of the edge stitches. This is a great way to use up lots of small scraps.