I think that in the situation described by the author of the topic in the start post, the causes and effects are reversed. People work hard not to play more or to have the opportunity to lose more money on slot machines. Here, the pattern is most likely the opposite. People work hard to earn a lot. But hard work is hard and stressful. To relieve stress, people simply play more. Therefore, I believe that the definition that people play for fun is not entirely correct. In this case, people play to relieve stress.
I also agree with @julien_olynpic that besides being a means of stress relief, games like slots also serve as a form of psychological compensation. For some people, when they've done monotonous and stressful work, they often don't feel like they're getting instant rewards. Here, slots offer instant gratification, a feeling of psychological satisfaction that's rarely achieved through hard work. So, I don't think people simply play for fun; they play because the game itself provides a quick mechanism for feeling a sense of control and satisfaction that's not found in the real world.