This OpinionJournal editorial lays it out. I especially like:
Of course, the idea of opening our market to imports has plenty of enemies–such as steel, textile and agriculture industries. And there is lots of political hay to be made by representing these interests. But the economics are clear: Imports offer American consumers more choice, higher quality and lower prices.
Importing competitive goods also means importing knowledge. The more domestic goods are exposed to superior foreign goods, the greater the transmission, or spillover, of know-how across countries. And that holds true not only for the U.S. but for the rest of the world. Countries with lots of trading relationships are in a better position to absorb the technological progress of more advanced nations. The result is more rapid productivity growth.
The flip side of restricting imports is subsidizing exports. But this, too, is a dubious economic notion. A country that cossets its export sector creates pockets of inefficiency. The concept was accurately, if wickedly, characterized 200 years ago by Frederic Bastiat, the French libertarian. Bastiat suggested that since exports were preferred over imports, the optimum thing to do was sink all ships leaving home ports–thus creating exports without imports.
The importation of knowledge and Bastiat, all in one editorial. Trade is by far the best conduit of technology from rich countries to poor ones, making them less poor. That’s the most incontrovertible lesson of economic history.