When it comes to being a great singer, it boils down to having subtle control of your vocal cords. The problem most people have is that the larger muscle groups that surround the vocal cords tend to squeeze in and prevent the vocal cords from doing their job.
The shoulders play a big role in freeing the voice, because when they become mobile and free when you sing, it can open up a level of freedom in many of the smaller muscles that are critical to a full voice. Most singers tighten their shoulders, neck, and jaw when they sing, resulting in a voice that is limited, particularly in the higher range.
On the performance side, a singer’s ability to communicate with their whole body can determine whether or not they connect with their audience. Phillip Phillips sang “Too Hot To Handle” on American Idol last week, and used his shoulders a lot to express the emotion of the song. Check it out here: