The Start/Stop button is one of my absolute favorite features about this machine. I use it for free-motion quilting, because I can't seem to get the hang of my hands and feet moving in tandem with each other. So I just set the speed control, press that button and I'm off and running! No need to be coordinated (LOL), I just concentrate on where my hands are moving the quilt.
It's also great for things like sewing long strips together. For that, you can set the machine at top speed and zip along after pressing the S/S button - no need to keep the "pedal to the metal" with your foot.