Raise your arms overhead. Go on, try it. Now holding them up there, straighten your thoracic spine, exaggerate “good” posture. Look where your arms are now! Magic? No. Just proving a point.
Your thoracic spine contributes significantly to your trunk, shoulder and neck mobility.
Without a fully functioning thoracic spine, your range of motion will be limited. This limitation will often cause a “loading” of different structures and ultimately can lead to an injury or pain.
Muscles doing a job they weren’t designed for, aren’t happy muscles.