If you plan to apply for a loan at a lender or dealership, you will need to provide "proof of ability to pay":

You must set up a policy beforehand and provide the binder or policy number at the time of purchase. Financing-Specific Documents

This serves as proof of identity and confirms you can legally drive the car off the lot.

Bring your existing insurance card or policy declaration page. Most insurers provide a grace period to add a new vehicle.

Documents To Bring When Buying A Car - Canfield - Haus Auto Group

Regardless of how you buy, these items are non-negotiable at any dealership or for legal road use: