This also raises your renown and can get you invited to feasts and will let you join the guilds, which is helpful for finding quests. You can get almost 30,000 gold in under an hour, assuming you can find enough tournaments in that time, Lastly your gear and level dont matter at all in the tournaments, so its great practice for learning the combat system and getting used to all the weapons without worrying about any penalty. Second, by gambling you can end up earning over 10,000 gold in every tournament. First tournaments are easy to win (except for swadia and their damn jousts -_-).
Tournaments are great for three reasosns. The easiest way to earn money fast in the early game is to go fight in the tournaments, and gamble on you winning.