The Crack In The Windshield
You might have been ignoring it for a while now. Maybe it started very small and has grown a little over time. Or maybe you just hoped that if you pretended it wasn't there and it wouldn't be a problem.
The truth is, a crack in your windshield makes it weaker, plain and simple. And the windshield is there to protect you, not just from wind, but from the detritus of the road. Rocks that have been kicked up or have fallen off of other vehicles are an ever present damage; even more so if you live in a hilly or mountainous area. A rock that might bounce off a whole windshield could go right through yours, which is not the desired outcome.
Also, as cracks grow and spread (and they have a tendency to do so, with the heat and cold cycle of the summer to winter months) they can begin to obscure your vision as you drive. You may not even have noticed it at first, it's such a slow change over time that you might not realize how much work you're doing to compensate for crack in your windshield. Try to think about it, next time you drive.
The other thing is that a lot of your vehicles safety features- how the airbag deploys, crumple zones, and how the vehicle distributes force from an impact- depend on the windshield more than you'd think. A damaged windshield mitigates those safety features and makes the car less safe in case of an accident.
Now, small cracks and chips can be mitigated somewhat. There's crack and chip repair kits you can get to fix up small, new cracks. In most cases they won't hold up as well work done by a professional, but if you just want to live your life and not have to worry about replacing the windshield this moment, that may be an option for you. When you park, try to keep the windshield out of direct sunlight. Park in a garage or under cover if at all possible, this will help keep the temperature extremes down somewhat.
And if you didn't get on it immediately, well, the older the crack is the more likely moisture and dirt has gotten in and expanded it, as well as the heat and the cold. If this is the case, the more likely it is to spread and get bigger, and eventually you'll probably have to replace your windshield.
If you have any size of crack in your windshield, contact a windshield replacement service in your area.