Many injuries are caused by loose objects that "fly" in a collision. When your vehicle suffers a sudden deceleration (as in a collision), the only things that stop with it, at least immediately, are the ones that are secured somehow. When the vehicle decelerates, a loose object temporarily keeps traveling at the speed the vehicle was originally going, until it hits something else in the slowing vehicle -- like you, maybe.
Even fairly light-weight objects can cause serious injuries. A tissue box weighing half a pound, at 60 mph, would feel roughly like about 30 pounds hitting you in the back of the head! I think that would hurt a little bit. Think about the damage heavier objects could do. Do you ever carry a bowling ball in your vehicle? What about canned goods in a grocery sack or two? The forces involved in more serious collisions can send these things flying like cannon shot -- and send you to the hospital.
Finally, inanimate objects are not the only loose objects that can hurt (or even kill) you. If I should fail in my responsibilities as a driver and hit something, or get hit by someone else, any passenger in my vehicle that is not belted becomes a loose object also. Drivers, safely belted into their seats, have been killed by un-belted back seat passengers who are launched forward in a crash. Babies, if not safely strapped into child-restraint seats, can be launched out windshields. Some parents think they can hold on to a baby in a crash. Because of the forces involved, that is often not the case; children can be involuntarily ejected from their parents' arms, despite their best efforts to "hang on." In my vehicle, everyone is asked to buckle up -- no belt, no ride.
Take a look around your vehicle. Make sure everything is in a safe and secure place, in the trunk, under a cargo net, etc. Make sure the only thing "flying" in your vehicle are the words to your favorite song!
Keep it between the fence posts!