The scapula, generally referred to as the shoulder blade, is an essential piece of all upper body function. To understand what the scapula does, you first need to understand what it is. Freely sitting on the back side of the rib cage, the only true attachment this bone has to the torso is where it meets the collarbone, right above the shoulder (this connection is called the acromioclavicular joint). As you can see in the picture below, this is not a very large connection. Why is this important you might ask? Because this means shoulder blade movement is not restricted by the structure of the joint itself. It's able to move up, down, right, left, forward, back, and more, all because it isn't limited by its connection to the body.
Okay, all of that's great and all, but how does that have anything to do with the shoulder itself? You also need to understand that the joint that you picture when you think "shoulder" is called the glenohumeral joint, and it's where your shoulder blade meets your humerus (arm bone). So wherever the shoulder blade goes, the shoulder itself follows, meaning the freedom of movement we discussed previously of the scapula allows the shoulder to move freely as well.
Like any joint, the scapula's movement is controlled by the muscles that attach to it. Not getting too far into the weeds, the muscles of the scapula allow it to:
1. Elevate (Move Up) - mainly controlled by upper trap, levator scap
2. Depress (Move Down) - mainly controlled by lower trap
3. Protract (Move Forward) - mainly controlled by serratus anterior
4. Retract (Move Backward) - mainly controlled by rhomboids, mid trap
5. Upwardly Rotate - mainly controlled by upper trap, lower trap, serratus anterior
6. Downwardly Rotate - mainly controlled by rhomboids, levator scap
As you could imagine, these muscles not only influence how the shoulder blade moves, but how the shoulder joint moves as a whole!
So for your shoulder to move fully overhead, your scapula has to rotate upwardly to allow it. For your shoulder to reach across your body fully, your scapula has to protract to allow it. Any motion you want your shoulder to perform, your shoulder blade has to move in conjunction to make it happen. And if it doesn't... problems are around the corner.
Let's take someone with difficulty reaching their arm overhead, for example. Maybe this individual has some tightness in their rhomboids (remember, downward rotators!), which leads to the shoulder blade being unable to upwardly rotate optimally. Once we relieve the tightness in the rhomboids and restore the upward rotation of the scapula, the shoulder's overhead motion will normalize.
This correlation applies to any movement limitation in the shoulder. So next time your shoulder seems to be acting up, take a closer look at your scapulae!