People should wear seatbelts because if you were hit by a car or any other object you could fly out of the window and you could die. Newton's first law was that"An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalaced force. If you grt in a car accedent and you were going about 70 miles per hour and if you crash then you will still go 70 miles per hour.
when not wearing your seatbelts
If you are not wearing your seat belt then you can fly out of the window and you wont see what happends unless you lived the hit. You can get badly injured and that you can lose a body part.
2nd and 3rd and 4th http://www.bing.com/images/search?q=Seat+Belt+Accidents&FORM=RESTAB