In a nut shell… most golfers have no idea that the Lower Body holds or resists on the backswing. This gives the Shoulders something to wind up against to create torque. If the lower body slides towards the back foot on the takeaway, there is no Anchor to wind up against. If you slide, your chances of sliding back to where you started are almost nil. Or worse, the hips are allowed to start the backswing (ouch), that is like the middle of a spring trying to wind itself.
The golf swing is much easier when you learn to resist with the lower body, while you use your shoulders to wind your arms and club around your spine. This takes some work and practice, but the result will be your downswing is awesome, powerful, repeatable, consistent and easy.