The loss of structural integrity of the vehicle
Federal Motor Vehicle Safety Standards set particular requirements for roof collapses in a car accident. The windshield is an integral part of that roof protection along with proper airbag deployment, to insure protection of the occupant in the passenger seat.
Damaged glass is 60 - 70 % weaker than undamaged glass and that weakness can lead to separation of the windshield from the vehicle.
Without the windshield, roof strength may be reduced by 30 % and can easily collapse
Far side roof strength is reduced as much as 30% by near side windshield breaks, plus as additional 40% by the more lateral loading
Federal Motor Vehicles standard 216 requires that in a roll-over crash the roof does not collapse more than 5 inches when 1.5 times the weight is applied to the A pillar.
The A pillar is that support between the windshield and the door opening. If the windshield comes out partially or completely, the roof collapses quite easily, leading to terrible injuries or even death.
Windshield’s cushion the blow for occupants
The glass may break, but the polyvinyl layer is flexible and cushions the impact. A windshield with structural damage will lessen it's ability to protect the driver during an accident.
Increased risk of ejection from vehicle
The windshield is the single largest opening in the vehicle; if the windshield is separated from the car in an accident, the risk of ejection increases. Again, a windshield weakened by a crack is more likely to separate from the vehicle. One of the main purpose of the windshield is to keep the occupants inside the vehicle. 30% of automobile accident fatalities occur when the occupant is partially or completely ejected from the vehicle
30% of auto accident fatalities occur when occupant is ejected from car
A safety feature in your car With separated windshield from the car, the risk of ejection greatly increases
The windshield is designed to effectively be the backboard for airbag deployment. When an airbag deploys, it uses the windshield to push the airbag toward the passenger. If a windshield is weekend by a crack, it may separate from the vehicle in an accident, leaving nothing for the airbag to work with to protect the occupant. The airbag may not inflate and may not be in the correct position for protection.
When the airbag deploys it uses the windshield to push it toward the passenger.
Without the windshield airbags may not inflate properly nor in the correct position.