It protects the adhesive
The polyurethane holding your windshield in degrades under ultraviolet light. The frit band is an opaque ceramic layer fired into the glass surface, and it shades the adhesive bead from UV for the life of the vehicle.
This is why the band is exactly as wide as the bonding area, and why a windshield with a scratched or damaged frit band is a problem rather than a cosmetic flaw.
It gives the urethane something to grip
Fired ceramic is rougher at a microscopic level than smooth glass, so the adhesive keys into it. A bead laid on clear glass instead of on the frit is a weaker bond.
The dots spread the heat — and hide the transition
The fading dot pattern is a halftone. During manufacture the glass is heated and bent, and a hard edge between opaque black and clear glass would create a sharp temperature difference and optical distortion right where you look. The dots blend the transition.
They also hide the edge of the bead from inside the car, which is the part you actually notice.
Need this done rather than explained? There is no shop to drive to. We come to wherever your car already is — your driveway, your workplace car park, an apartment lot, or the side of the road it stopped on — anywhere in greater Sacramento, with no call-out fee and no mileage charge. Give us the year, make and model and you will have a firm price before the call ends.