Yes, the shoulders don't ever seem important when you just think about them.The thing is, even if we don't look directly at the shoulders, every time we look at the photograph or an image, we use a quick view of the shoulders to sum up a lot about the character of the picture.
You see, the shoulders or the widest object in the picture. They help bring focus towards the face. That's why the shoulders are important. To actually get this angled effect, it would be an idea to get the person you're taking a picture of to lean on the wall or something. You could also ask them to stand on one foot too, and then lean on something. This kind of pose will automatically help you get into the right kind of cocked shoulder position. As an amateur portrait photographer, you need to get a bit of practice in with this.