Replacing car glass is necessary depending on the size of the crack. Small chips can easily be repaired and the the windshield will still be quite safe. When it comes to cracks, the length and depth of the crack will determine if the windshield can be repaired or if it needs to be replaced. A crack that is less than 3 inches long and fairly shallow can be safely repaired. A deep crack or one that is longer than 3 inches can be repaired, but the windshields’ integrity cannot be guaranteed. It is better to replace the windshield in these circumstances.
For more info, see Average Car Window Replacement Cost