Even a car that is being stored has certain risks, such as vehicle theft, vandalism, and liability concerns. If you do not insure the vehicle and something happens, you will be held financially liable, and the costs could turn out to be far more than the cost of an insurance policy would have been. Since even a low cost repair is going to be more than you'd pay in premiums for a long time, having the insurance coverage makes good financial sense.
Theft and vandalism coverage are important for a vehicle that is being stored. Because it is not being used daily, miscreants may see the car as an opportunity. Burglary and theft are common in such cases, and expensive or rare vehicles are tempting for quick sales. If the car is important enough to store, it should be important enough to protect, as well.
Additionally, most states require you to have a current registration for all vehicles in your possession, whether they are being driven or not. Since a requirement of registration is to show proof of insurance, you cannot legally register your car unless it is also legally insured under the laws of your state. Failure to insure a registered vehicle would result in the registration being revoked, and could lead to the suspension of your driver license.
There are less expensive insurance options available for cars that rarely driven such as pleasure use insurance. Some companies also offer pay-by-the-mile insurance, for example, while most companies offer reduced rates for leisure vehicles that are driven a limited number of miles each year. Your car might be better off insured in this manner, and it would be a lot cheaper than buying regular car insurance.