If both sides did not benefit, it would not continue to happen. It is precisely because commercial exchanges leave both sides better off that we all engage in them, and trade is just such exchanges across national boundaries. Which is why countries have a common interest in trade and common rules for same. It is all about mutual gain and mutual benefit. Which is why the protectionist policies of the 1930s were not only disastrous in effect, they were criminally stupid.
More wealthy countries mean more people able to buy more of our products.
The WTO is a way for governments to mutually agree not to do stupid things. Or, to be a bit more precise, to mutually agree to each defend their own general interest against their own special interests clamouring for special privileges. With the current WTO agenda, Australia is in an enviable position. There was no significant local interest which was threatened by anything on the agenda, but, as a major food exporter, we stood to gain a great deal from liberalisation of agricultural markets. Something else which stood to gain a great deal from such liberalisation is the environment.