Well, it's a free-trade agreement. Mutually getting rid of tariffs increases trade, which makes all the participating countries better off.

In addition, it tries to remove non-tariff barriers to trade, which involves harmonizing legislation. E.g. if you want to sell US beef in Japan and Japanese beef in the US, then the health regulations about how cows should be raised should be the same in both countries.

The copyright stuff comes under the heading of harmonizing intellectual property law, although somehow it seems to end up as pro-copyright-holder as possible...