Without that tiny button, the latch could slide all the way toward the floor or become trapped between the seat and the center console. That would make it harder to grab each time someone gets into the car. By keeping the latch at a convenient height, the stopper makes buckling up quicker and more comfortable for both drivers and passengers.
The button does not directly protect someone during a crash, but it supports proper seatbelt use by keeping the latch easy to reach and helping the belt remain neatly positioned. Some vehicles may also have more than one stopper to limit how far the latch can move in either direction. If the button breaks or falls off, the seatbelt can usually still function, but the latch may begin sliding freely along the strap.
Because the stopper is such a small part, many people only realize what it does after it is missing. Replacement buttons are generally simple components designed to restore the latch to its normal position. It is a good example of how a tiny feature in a car can make an everyday safety system easier and more convenient to use without most drivers ever noticing it.
