The answer for this is simple. Let me give you a few points.
First, there is a scarcity of natural resources. We can feel this even today. The price of commodities always seems to go higher. Consider the price of oil. Two years ago it hit a price of 150 dollars a barrel. Why? Partially we are running out of oil. In short, nations always want more commodities, because there are in limited supply.
Second, in order to build an empire, you need natural resources. So, there are domestic needs in empire building. For example, you need copper, metal and the like to build infrastructure, buildings, ships, and the like.
Finally, the trade is raw materials is profitable. You can refine raw materials and then sell them to other developing nations for a profit.