Seatbelts are designed to protect passengers by preventing them from being thrown forward due to inertia during a sudden stop or collision. By restraining the body, the seatbelt applies a force over a longer period, reducing the magnitude of the force experienced by the body (as per the impulse-momentum theorem, FΔt = Δp, so if Δt increases, F decreases for the same Δp), thus minimizing injury.