Just for fun. I don't know about others, but I considered gambling as a stress reliever. There are many reasons why people gamble, but money tends to be their main motivation. While many of us may think that the only reason to play it is to win huge amounts of money, I disagree.
If you pay close attention, you'll see that less developed nations have more gamblers overall, despite their lack of money. They can't even effort it but they are addicted to it. They just care about money, money, and more money.