Because it could determine if a repair or replacement is necessary.
If the impact is located in the driver’s line of vision, we might be able to repair the damage; however, since this would affect your visibility, the safer option is likely to replace your windshield.
Also, if the impact is located too close to the body of the vehicle, we won’t be able to use the equipment needed to repair the damage, and the windshield will have to be replaced anyway.