In high speed collisions, Inertia can often be fatal.
Inertia is an objects reluctance to change direction or alter it's motion. So it has direct connotations with Momentum. If you are moving in a care at 120mph, you are also moving at 120mph as observed by someone in an inertial frame of reference. In a crash, both the car AND you need to be stop. The car is totalled due to it's design (crumple zones and such), but you (a fleshy construct) will not be stopped until ALL of your momentum has been transferred. Your high inertia which is dependent on your mass and the speed at which you are moving will make this difficult and painful.
Now transfer of momentum requires a force. To get rid of your momentum a change in momentum (which is a force must be induced). So without a seatbelt you would crash into the windscreen, and due to Newton's 3rd Law, the windscreen would induce an equal and opposite force back on you. So you would be crushed to death.
It gets messy with the explanations when you start considering Kinetic energy also.