Longarm Quilters like for their customers to provide extra batting and backing for many reasons. One of the reasons is because our quilting foot hits the grippers holding the sides of the quilts, so it is very difficult to stitch within 3+ inches of those grippers.  However, for me, the biggest reason we like extra fabric  is because the backing and batting draw up during the quilting process. Another reason is so that we can accurately provide good stitch quality and tension for your quilt. Longarm machines are so finicky when it comes to tension; much, much, much more so than a domestic machine, that it can be difficult to achieve good tension and good stitches. So, most of us will test out the tension and stitching on a side piece before we add it to the quilt.  If you look closely at this picture, you will see the side border of the quilt I was working on.  There is about 6 inches of extra backing and batting.  I added a 3″ wide piece of fabric  at the side of the border and tested my stitches there with each thread change before stitching on the actual quilt.