Yes you can get insurance on a car without registration since when you register a car to get the license plates you'll need to provide proof of auto insurance before you get the license plates for you car.
So yes you can have auto insurance on your car without having registration but if you're caught driving without a license plate you'll still get a ticket or your vehicle towed even if you have auto insurance.
There's really no other reason to get auto insurance on your car unless you plan to register it with the DMV and drive it.
But you can indeed have auto insurance without registering the vehicle.