The shape of the golf swing is a circle. In order to create the circumference of the circle, one arm needs to be straight or extended both during the backswing and the follow through. For a right-handed golfer that means keeping the left arm straight or extended on the backswing with the right arm bent or folded and keeping the right arm extended on the follow-through with the left arm folded. This is how we create the circumference of the circle. I see a number of people who bend both arms at the top of their backswing which is a weak position. It will be easier for a golfer to make solid contact with the golf ball by keeping one arm extended on both sides of the swing because the club needs to bottom out at the golf ball to get underneath the ball to shoot it in the air and this happens more often when the left arm is extended at the top of the backswing. When a golfer bends both arms, it's harder to be consistent because the golfer needs to then extend both arms on the downswing to even make contact with the ball in the right spot. The tendency I see when both arms are bent at the top of the backswing is a fat shot - the club hitting the ground before the ball - and it's usually a really deep divot. Ideally, we want the handle of the club to lead from the top of the backswing and the head of the club to follow. This is really difficult to do when the left arm is bent.
On the follow-through the opposite occurs. The right arm should be extended or straight and the left arm should be bent or folded as the club moves through impact towards the shoulder. By creating a better circle - crisper contact should occur more often.