The NAFTA agreement benefits the US economy because it creates a free trade zone that comprises the territories of the US, Canada and Mexico. Outside the US borders but inside this zone, US products compete in equal conditions with the domestic production respectively in the Canadian and Mexican markets, enhancing US exports and total GDP.
Moreover, it also allows products from these two countries to enter the US markets and to compete in equal conditions with the domestic producers, which at first might be regarded as a drawback but it is not, because stronger competition forces US producers to keep on investing money on innovation and improvements in order to beat their rivals and attract consumers. Through this process, competition leads to more efficient industries which are able to produce at a lower cost. Finally, consumers are benefited too because they can acquire products at a cheaper market price.