To create your maximum clubhead speed from your golf swing, you want all your muscles to move through their full range of motion.  Your shoulders, especially, should move freely throughout the swing.  Your arms should swing independently from your chest, but if your chest muscles tend to be overly tight they actually start to take over the swing limiting your shoulder turn.

Exercises that loosen and open the chest allow you to swing your arms more freely so that you create a fuller shoulder turn.