The concept of presettting your shoulders is a fairly new idea. (or at least the popularity of the concept is fairly new.) It came about with changes in equipment. In the old days nobody needed to play ultra-deep on the lane. Now bowlers, especially the pros, play so deep they loft the left gutter. If you draw that kind of ultra-deep shot back on the lane, you run out of room. On the right lane in most centers you'd need to stand left of the ball return in order to create the angle. To compensate, styles changed and bowlers, from the walk to the target point-of-view, commonly fade left to throw right.
The downside to presetting the shoulders is that your swing plane is changing as you move on the lane. This can cause timing and balance issues. Its a big part of the reason crankers are stereotypically considered less accurate. But if you are going to play inside, then its a required technique to master.
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Good luck & good bowling