This shows us getting wealth is tied to traveling the earth. The value of things changes based on their location. Take the example of a city which produces a lot of rice and little to no wheat. People who live here have more rice than they can eat, and anyone can buy a sack full of rice. For one silver coin in the city.
Now, imagine you took a thousand sacks of rice, filled an entire ship with them, and sailed to a far away city bearing the pain of travel in this far away city. Wheat is local produce and rice is a delicacy. And people buy one sack of rice for one gold coin. So you sell the rice in the city and load up your ship with a thousand sacks of wheat and sail back, making lots of profits.