It gives you more gold from your city, I think 50%. But the amazing thing about them is they make more citizens happy. A city without a marketplace, luxury resources will make one unhappy person content (or one content person happy, can't remember), for each luxury resource. If you have a marketplace, then after every 2 luxuries you have it will make one more person content. Like if you have 3 luxuries w/o MP you get 3 content ppl, with a MP you get 4. 5 luxuries w/o MP = 5 content ppl, 5 luxuries With MP = 9.
Build a marketplace in a city and make sure you have a couple luxuries i think a bit to the left near the top half of the screen it shows many ppl are content from luxuries, it shows it better than I can explain.