The cars of today are highly complex and technical compared to what was being produced just a few short years ago, not to mention expensive. Front and side airbags depend on the windshield and side glass for support during an impact.
Structural designs depend on a properly installed windshield and back glass for support.
The use of improper urethane can be both unsafe and harmful to parts of the electrical systems, and some expensive airbag systems can be easily set off during the process of replacing auto glass. The metal area of the auto that the windshield sets in is named the pinchweld. In the process of removing the broken windshield and old urethane, this metal may be heavily scratched. In order to prevent the pinchweld from quickly rusting out, the area must be re-primered. Windshields must be properly prepped and cleaned in order for the urethane sealant to properly bond to the glass.