> you should be asking yourself why a free trade treaty is so important when we already have free trade

Depends on your definition of "free trade", but we don't have completely free trade; many countries have high tarrifs, for example Japan's rice tarrif.

http://blogs.wsj.com/japanrealtime/2015/10/06/tpp-deal-expec...

(OK, this is TPP, but I'm sure the same point applies).