According to Canada Safety Council:

"Your vehicle's windshield is more than simply a window to let you see where you're driving. It is a critical component of your car's safety system. If it has been replaced, improper installation, poor quality glass or the wrong adhesive could cause serious problems.

The windshield positions the passenger side airbag. If your windshield becomes dislodged because it was improperly installed, the airbag cannot protect the passenger as it should. It also helps support the roof. In fact, the windshield is integral to the structural integrity of the roof.

Urethane, the black rubber-like substance you see around a windshield, is also part of the overall structure of your automobile. It must be strong enough to hold the windshield to the surrounding body. Its strength increases as the new urethane cures and reaches the adequate strength after several hours, depending on the product used. Butyl, silicon or other caulking is not acceptable."