Why is trade between countries important?

Imagine a scenario where there is no trade between countries. This would imply that:
1. Each country would have to produce and consume the entire gamut of products and services that are required by its people. As a cororally, citizens would be able to demand only those things that the country is capable of producing.
2. Speaking in terms of economics, countries wouldn't be able to focus on production of things in which they have a competitive advantage (a country has a competitive advantage in something which, it can produce with the least amount of resources/at the least cost relative to other nations)
3. Countries that have a rough terrain or are rich in only a specific form of resource (think about oil rich middle eastern countries) wouldn't be able to sustain their populace. Also, in the absence of technology, they won't be able to use their resources efficiently.
4. There would be a lot of unnecessary duplication of investment in capital goods such as factories, technology development, etc. across countries.
5. There would be no multinational corporations as they would never get access to  huge markets or get a chance to take advantage of economies of scale in production, marketing and distribution.
6. No trade/dependency between nations would make resource wars/ forced colonization/ slave labour more likely.
7. In the absence of investments and technological inputs from developed countries, emerging and underdeveloped nations would take that much more time to catch up.