Trade keeps economy open, dynamic and competitive.  
https://ustr.gov/about-us/benefits-trade --- to pick just one comment off the internet.  
But in a nutshell, when you don't have free trade, Canadians end up spending more for products in protective industries -- just one example of many, have you looked at the price of cheese or milk in Canada compared to the rest of the world when common sense tells us with all of our agricultural land why does this happen.