In most of the world, vehicles are subject to the World Forum for Harmonization of Vehicle Regulations's standards, but the US has refused to recognize the guidelines. Instead, US cars must comply with the Federal Motor Vehicle Safety Standards, superficially different as to make compliance with both impossible.
Many automakers have exited the US market or never entered due to the high cost of conforming to differing standards. This reduces both innovation and consumer choice. In addition, smaller US companies must spend extra to sell their cars internationally. Many vehicles sold internationally are not even available in the used market.
If the US allows WFHVR-approved vehicles, then automakers can save money, innovation can increase, and new contenders can more easily enter the market.