Using both seat-belts and shoulders straps is important because they:

Seat-belts give support in cars, buses, trucks, etc because if you get in a crash (an accident), they hold you back and don't let you hit your head or fly off the car. Shoulder straps help you for when you are riding a bike, scooter, or rollerblades, you won't hurt your shoulders when you fall. 
Hope I Helped!

Generally from flying out the front windshield or falling in the floorboard and getting crushed and whatnot, mostly so you don't leave your seat during the wreck