OK, as promised .
I hope you don't mind but I'm going to split my reasoning over two (or possibly more) posts. There are (at least) two strands to my argument, and they are quite involved.
So. TTIP is about liberalising international trade. My first post explains why I support international trade and reducing the barriers that restrict it.
My starting point is that I believe that capitalist economic systems are the best (or least bad) available to us. They're not perfect, for absolute sure, but they're better than any of the alternatives. I like being able to chose the products I buy and the shops I give my business to. And I think that economic choice increases efficiency because weaker businesses improve or go under while good products and businesses do well - and that increases wealth for the population as a whole. (I'm very happy to debate how wealth should best be distributed and if necessary redistributed among the population in a separate thread; it doesn't make any odds for the purposes of explaining why capitalism is OK.)
The argument for free international trade is based on the theory of comparative advantage. To see how it works, imagine two countries, East and West, which both produce two kinds of goods, bicycles and wheat. In a year, an Eastern worker can make two bikes or grow four bushels of wheat. A Westerner, however, can manage only one bushel or one bike. Each country has 100 workers, and initially both of their workforces are split evenly between the two industries. So East produces 200 bushels of wheat and 100 bicycles, whereas West produces 50 bushels and 50 bikes. In total, the world produces 250 bushels of wheat and 150 bikes.
Since East can produce both wheat and bicycles more cheaply than West, it has an absolute advantage in both industries. Even so, Easterners will benefit from trading with Westerners. This is because East is relatively more efficient at growing wheat, where it is four times as productive as West, than it is at making bikes, where it is only twice as productive. In other words, it has a comparative advantage in wheat. At the same time, West has a comparative advantage in making bikes, even though it has no absolute advantage in anything.
Both countries will be better off if each specialises in the industry where it has a comparative advantage, and if the two trade with one another. Specialisation increases world output. Suppose that East specialises in wheat growing, shifting ten workers from its bicycle factories to its fields. It now grows 240 bushels but only 80 bikes. West moves 25 workers from wheat farming into bike making, where its comparative advantage lies, and now produces 75 bikes and 25 bushels. Global production rises - now the world collectively is making 155 bikes and 265 bushels (instead of 150 bikes and 250 bushels).
The point of economic activity, however, is not to produce but to consume. Both countries can enjoy more bikes and more wheat if they trade on terms at which both will gain. If East is going to import bikes, it will pay no more than two bushels in return (if the price were set any higher then it would be better off moving workers back to the bike factory). Similarly, West will pay no more than one bike per bushel. If the price ("terms of trade" in the language of international trade agreements) are set at one-and-a-half bushels per bicycle, then both countries are happy. 33 bushels are traded for 22 bikes. The result is that both countries are better off.
Economists' next argument for free trade is that opening up markets to foreign suppliers increases competition. Without free trade, domestic companies may have enjoyed monopolies that enabled them to keep prices well above marginal costs. Trade liberalisation will undermine that market power. Competition should also spur domestic companies to greater efficiency because they will not be able to pass on the costs of inefficiency through charging higher prices.
In addition, free trade means that firms are no longer limited by the size of their home country, but can sell into bigger markets. In industries where average production costs fall as output increases, producing economies of scale, this means lower costs and prices. In such industries, trade also increases the variety of products on offer. If a car manufacturer, say, were limited to its home market, it would have a choice between producing small quantities of a number of models and large quantities of just a few, which could be produced more cheaply thanks to economies of scale. But given free trade, it would be able to produce more models because they could all be produced in large enough numbers.
Incidentally, the internet has been revolutionary in opening up new markets for micro businesses - and micro businesses benefit most from removing barriers to international trade because they are least well equipped to manage trade barriers.