Setting up with the ball off the heel of the clubface encourages an in-to-out swing path through impact, as you feel like you have to reroute the club closer to your body to find the center of the clubface. This is a great visual cue for correcting the out-to-in swing path that produces pulls and slices. In fact, look carefully and you'll notice many Tour players address the ball off the heel. Try it and see if your shots straighten out.