A small chip in a vehicle's windscreen is probably easy enough to fix; many repair companies can even visit your home or office and fill in that chip with a type of resin or gel that hardens and makes the damage virtually invisible. However, when a windscreen has suffered more extensive damage, and even if the damage is small, you might consider replacing the glass altogether. Note when it's good to have a new windscreen installed rather than attempting to fill a chip or crack, and why this may be a better option for your vehicle.
It's been damaged before
The gel or resin that is used to fill in a small crack or chip is strong enough to hold the windscreen together, but not as strong as solid glass. If you try to fill too many chips and cracks with this resin or gel, this can result in a windscreen that is not very strong. You may not think that a strong windscreen is important, but the windscreen actually holds up the roof of a vehicle in the event of a rollover accident, and it keeps you from flying out of the vehicle's cab in case of a collision. If the windscreen is weakened from too many repairs, your safety might then be compromised, so have a new windscreen installed instead.
Windscreens are meant to withstand heat and cold, but extreme temperatures, including direct sunlight and freezing rain, can eventually compromise the integrity of the glass. If you live in a very extreme climate, you might consider getting an older windscreen replaced when it's damaged, just to ensure it's strong and durable and will withstand a collision. This can also mean avoiding future damage from flying debris.
The gel or resin that is applied to a windscreen to repair it doesn't dry perfectly clear, so that it may leave a slightly cloudy finish. If the crack or chip or other damage to the windscreen is in the driver's line of vision, you may not want to opt for repairs, even if this work would fill in a crack or chip. Even a slightly obstructed view can mean a greater risk for an accident, and especially when driving at night, or if direct sunlight should hit that repaired area and cause a glare. If the repair would interfere with a clear view through the windscreen, have that glass replaced rather than repaired.