But, of course, we don't live in that world - we have to direct resources where they will be used most efficiently and productively. And at the end of the day, that's what free markets do - allocate resources to their most productive use.
It's all about comparative advantage - autarky (trying to do everything yourself in the name of self-sufficiency) is hugely inefficient when compared to trade in good and services. It seems totally counterintuitive, but it's true - concentrate on producing the things that you have a comparative advantage in (even if you don't have an absolute advantage), trade for the other things with folks that have some comparative advantage in those other things, and you all get richer thereby.
It's true for you as an individual, if you think about it - you could produce everything you need to live, and thereby be completely self-sufficient. You know, growing your own vegetables, hunting for your own meat, making your own weapons to hunt with, making your own tools to garden with, building your own shelter with materials and tools you gathered yourself, et cetera, et cetera. But you, naturally, don't do that - you recognize, whether consciously or not, the value inherent in concentrating on producing the things you're good at producing. That is, doing whatever you do for a living, and trading for the things you're not so good at doing.
After all, consider me - I'm terrible with plants. They seem to turn brown just by being in my presence. If I had to be self-sufficient and grow my own vegetables, I'd be starving in very short order. I'm just not very good at it, and it's wasteful and inefficient for me to try - my labor would be better directed to doing something else. So, instead, I let the people who are good at it do it for me, and trade with them to get vegetables. This frees up my time, so that I can produce things where I have some comparative advantage. It's good for me, and it's good for the farmers - everybody's happy, and everybody gets richer than they otherwise would.
And it's the same for nations as it is for individuals. Comparative advantage isn't called Ricardo's Law just out of economists' arrogance - it's a mathematical certainty. Nations producing the things they have a comparative advantage in, and trading for the rest makes everyone richer than the alternative. And that's the point to making things anymore, isn't it? ;)