The STS mode guides the putter on a straight back and straight through path. Simple, just like a pendulum.
The STS mode keeps the putter face square to the target line throughout the stroke. Face angle at impact is the greatest determining factor in getting a putt started on the target line. Keeping the face square to the target line throughout the stroke drastically improves the chances it will be square at impact.
The STS mode swings the putter in a true pendulum arc, which minimizes hand action and stroke breakdowns.