It is important to do exercises that develop rotational mobility. Firstly, if you lack mobility in that upper spine, there will be restrictions on how far you can turn your torso, therefore limiting your backswing.

Secondly, the lack of mobility in that upper spine will limit the backswing causing swing faults that could lead to swing compensations like swinging over the top, for instance.

Thirdly, without the ability to separate the upper from lower body, your swing will be limited in speed and power, therefore limiting your swing speed and distance.

In short, doing exercises, the ones in this workout, will help you increase the mobility in your thoracic spine so you can learn to make a better turn and get you more power and distance out of your golf swing.