I have no idea whether seat belts are actually a good idea.
I do know. And I believe in them. Here is the straight skinny.
All fatal collisions are not really one collision. They are three collisions. The first one happens when your car hits the other car. The metal starts to crumple and deform absorbing energy in so doing. The second one happens when your head and chest hit the vehicle interior. This usually won't kill you. The third collision is the one that kills. It's when your brain hits the inside of your skull and your heart hits the inside of your rib cage.
Seat belts work in combination with the structure of the vehicle. Together they slow your deceleration down spreading it across a greater distance so that the peak rate of deceleration is lower.
If you think about a watermelon travelling along at say 80km/hr and suddenly hitting a block wall, it's no surprise that it shatters and spreads Mellon guts out in a huge SPLAT. But if you start slowing the Mellon down from 6ft away such that it has stopped or essentially stopped before it hits the wall, it remains intact.
Think of it like the difference between a car with brakes and one with none. The more you can stop moving before impact, the better your chances.
Seat belts, their anchors, and the vehicle structure they are attached to, all work together to slow you down before your body hits the inside of the vehicle and long before your brains and heart go Splatt.
They work. Simple as that.
They are not perfect though. Faster speeds, poor brakes, impacts with stronger heavier objects can significantly reduce their performance.
I wouldn't leave the driveway without buckling up.