Most of the time the variation of your seam lines occurs at the end of the seam. The way it feeds in, you can't get a hold of enough of the fabric to keep guiding it in properly. By butting fabric pieces up against each other, it seems to eliminate this problem. Maybe because it is like sewing one continuous seam? The next piece helps feed through the last? I am not sure, but it really helped my accuracy :D:D:D
Another thought is, that it saves on thread as long as your scraps are not very big. Usually you cut off several inches of thread at the end of each seam, you use less using this method. My scraps are about 1/2" wide, or less.
Theoretically, you leave one under your presser foot at all times.
Also, using blocks as starter pieces, I can work on another quilt at the same time. Like in the pictures, I have a stack of blocks sitting next to my machine to work on. These blocks can be any size or type.