The most obvious reason is to financially protect yourself in the event of an accident. Car insurance can help cover the cost of damages to your car, as well as any medical expenses or property damage that might occur if you get into an accident.
Many states require drivers to have a certain level of coverage to drive on the road.
Finally, car insurance can also help you protect your assets. If you get into an accident and are sued, car insurance can help cover the cost of defending yourself in court.