Build the circumference and width of shoulder muscles
The barbell shoulder press is an action that truly targets the deltoid muscles. It concentrates the stress points on the target muscles.
The behind-the-neck barbell press transfers some of the weight to your latissimus dorsi, rhomboids, teres major, infraspinatus, trapezius and other upper back muscles.
In other words, your back is bearing some of the weight for your deltoid muscles. More training + more food = more muscle.