Without the button, the latch could slide toward the floor or become trapped between the seat and center console. By keeping it at a convenient height, the button makes the seat belt easier to grab and buckle each time you get into the vehicle.
The button itself is not responsible for tightening the belt or protecting you during a crash. Those safety functions are handled by other components such as the retractor, locking mechanism, and load limiter. Its role is mainly convenience, although making the belt easier to use can indirectly encourage proper seat-belt use.
If the stopper button breaks or falls off, the seat belt can generally still function, but the latch may slide farther down the webbing and become annoying to retrieve. Replacement stopper buttons are inexpensive, but any damage to the actual belt, buckle, or retractor should be inspected professionally.
