It is not decoration. That piece is a guide, sometimes called a stopper, and it plays an important role in keeping the seat belt correctly positioned across your body at all times.
Its main function is to maintain proper alignment. The belt should rest across your shoulder and chest, not your neck, where pressure in a crash could cause serious injury.
By holding the belt in place, the button also improves comfort. When a belt feels wrong, people are more likely to wear it incorrectly, reducing protection and increasing the risk of harm.
