Under normal conditions you would not want to cut your seatbelt.

There may be a need after you have been in an accident. Cars are deformed in accidents as well as the contents shifting around. It is not always possible to get to the the buckle after a collision.

If the vehicle becomes submerged, people often panic and forget how to unbuckle their seatbelt.

You would cut your seatbelt of the seatbelt of someone you were assisting to be able to remove them from a vehicle. This may be important if the interior of the vehicle is on fire. Do you want to stick your head into a burning vehicle to be able to reach the buckle on the far side? Easier to cut the belt and drag the vicitim out.