As a previous poster said, the benefits are all due to specialisation. If we simplify it right down and imagine three people stranded on an island we can see the benefits. Imagine they need to find food, collect water and build shelters. If each person tries to all three, they won't get good at any of them and it'll be harder work than if one person finds the food, one person collects the water and the other builds the shelters.
This is all globalisation is - just on a massive scale. As the costs of globalisation have decreased (eg transport costs) the benefits become more worthwile, and we see more specialisation and trade than we did 100 years ago.