Your credit score is used for auto insurance rates because your credit score shows how responsible you are and how good of risk you are.
If you have a higher credit score then you'll likely pay a lower price for your auto insurance but if you have a lower credit score or a really low credit score then you'll likely pay a higher price for your auto insurance.
When you have a high credit score you're considered a good risk and when you have a lower credit score you're considered a bad risk so your premiums will be higher.