Why, apart from their job was fighting in wars and jousts was good training but it was also a way to make a lot of money! many jousts with big melees or massed fights in teams allowed opponents to be captured and ransomed. Even if that was not allowed there were often valuable prizes and for unknown knights and those without a holding it was a good way to show good you were making it possible to gain a position in great lords household. So knights went from poor landless knights to in one case the Marshall of England, based mainly on his fame and wealth from jousting.