If there is any tightness in your shoulder area, you will not be able to maintain the extension in your arms during your golf swing. Your arms will bend at the top of your backswing and this will limit the amount of power you can generate.

Stand in upright posture with your arms hanging down comfortably by your side.
Reach behind your back with your right arm and touch the lower part of the opposite shoulder blade with palm facing out.
Hold position for a moment and return to the start position.
Repeat motion with the other arm.
Do 8 to 10 repetitions on both sides.