Function of seat belt
The purpose of a seatbelt is to hold the passenger in place so that he/she is almost a part of the car and to prevent him/her from flying forward if the car stops abruptly in case of a collision.
 When a car stops suddenly due to collision with another object such as another car, a tree, guard rail, divider etc. the car's acceleration drops very suddenly within no time. This is called deceleration. It follows the Law of Inertia which states, an object in motion continues in motion with the same speed and direction unless acted upon by an unbalanced force. As the car collides with another object, the other object provides the force which changes the speed and direction of the car. The car stops going in the direction it was going and throws the unbuckled passengers to the front. This is when a seatbelt comes into play. As the person continues forward with the same speed that the car was actually maintaining, the seat belt restrains him, holding him back from flying through the air, smashing through the car’s windscreen. The unbelted passengers may also hit the car's dashboard or steering wheel in absence of airbag, causing a heavy injury.

How does a seat belt work
A seatbelt has two parts. The first part rests over the passengers’ pelvis and the second part rests over the shoulder and across the chest. When the car stops abruptly, the seatbelt holds the body back and restricts physical injury.
When accidents occur, several systems incorporated in the seat belt assembly go into operation simultaneously. In less time than it takes to blink an eye, the seat belt system goes into action. By retracting some webbing as the collision occurs the pretensioner of the seat belt becomes operational and helps to restrain the passenger. The load limiter absorbs and softens the load on the occupant as the passenger moves forward because of inertia.
So it is not only due to traffic regulation that demands you should wear a seat belt, it is because of your own safety, you must put on the habit of buckling yourself. You never know when an accident will occur and it may be the seat belt which makes all the difference between you being dead or alive.