You're Correct!

When you address the golf ball your left arm should be straight. You do not want to lock it tightly and tensely, but you do want to achieve a fully extended position with your left arm.

Your address position is basically the position that you are wanting to get back into when you strike the ball. The only difference between your address position and your impact position is that your hips will have rotated more towards the left and towards the target if you are a right-handed golfer, with your weight being slightly more on your left side and your right heel being up off the ground and rotated slightly towards the target. Your left arm should return back to the ball and move through impact in a fully extended position.

Extending your left arm fully through your back swing and down swing will allow you to create the widest possible swing arc. This simply means that the club head will travel on the widest circle around your body. This is really important if you want to generate maximum power, as the widest swing arc allows you to swing the club head for the maximum amount of time to generate speed. This speed is then transferred from the club head into the ball through impact and as a result it generates distance for your golf shots.
Returning your left arm back to impact in a fully extended position also then allows you to extend your right arm through impact and this action adds further club head speed and distance to your golf shots.

Furthermore, extending your left arm at address sets the distance between your shoulder position and the golf ball. If your left arm was bent as you address the ball, then you would need to tilt over more and your posture would become incorrect, making it much harder to swing the club on plane and achieve an effective golf shot.

For a crisp strike and to achieve the maximum distance to your golf shots, address the ball with a straight extended left-arm.

If you set up in your address position and your left arm is not fully extended, then you wouldnt generate the widest possible swing arc. With a smaller swing arc, the club head will not travel as far around you and as such you will not hit the ball as far, as you will not have as much club head speed.

Having a bent left elbow at address will result in you having to tilt forward more than necessary to allow the club head to be set on the ground. If your posture is bent forward too much in your address position, you will find it extremely hard to keep your club head on plane throughout your swing. As a result your golf shots will become very inconsistent.

If you set up with the correct posture but your left arm is bent at your elbow, with your elbow bent towards the target, the club head of your golf club will be pulled up from the floor. If you return back to this position at impact, you will strike the top of the golf ball and produce a low shot that travels less distance than usual.