This question gets asked frequently for a few reasons:
To verify if you have coverage currently and your coverage history. This helps car insurance companies to determine if there are any gaps in your coverage or claims that may affect the new policy.
The car insurance you currently have can provide insights into your profile, the insurance company uses this information to determine your premiums they get insights into your risk profile. For example: If you´ve had continuous coverage with no lapses this may tell them that you are responsible and a lower risk to ensure. Additionally you will get better prices for your coverage.
Car insurance companies want to ensure that you are complying with the law since it is mandatory in most states for your car to be insured. If you have been driving without car insurance you will be considered higher risk and this will not be good for your quotes.
They may see if you have multiple insurance policies and the insurance company may offer you discounts for bundling your coverage and you may be eligible.
Car insurance companies ask about your current coverage to understand your risk profile, provide accurate quotes, and offer any discounts that may be available to you.