Since The Thread Director changes the way thread comes off the spool, your thread no longer twists as it comes off the spool. As a result, your thread lays flat reducing bulk in the seam. Seams are more uniform in length, which in turn reduces the need to stretch one block to match up to another. Your entire quilt will lay flatter making the quilting process much easier.
Featherweight owners will especially like the way the thread feeds into the machine smoother and more evenly.