The markets serve two purposes as I see it. One, to give the server a p2p trading system. Menchers mentioned above that this gives reason to settle within range of a village. The ability to trade with and interact with other players is the reason I don’t just play offline. I have used this aspect of the market to great success selling everything imaginable and have bought hundreds of items from others also. A better explanation to how to use the markets is somewhere else on this forum.
Two, Menchers called it a poor man’s void chest. I call the void chest a tiny market. The use of the inventory to store items cannot be underestimated. When I moved from Himinnfjell to Yggdrasil, I had thousands of items in the market. If not for the ability to move items in mass, I probably would have quit the game after Himinnfjell burned. Because of the ability to move all my storage in one go, I was able to build something that exceeded what the first was. Exploration will always be a part of a game like this, but the ability to store your treasures in the market, as a kind of safety deposit box, while you are out exploring greatly enhances the experience.
Just my two cents….