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.
Basically gambling is supposed to be for fun but for some persons who have turned it to a money making venture which has in turn taken money from them then it has literally given them but because they are bent on getting the money, most of them do not mind and will keep playing regardless of how much they have lost gambling.
The drive of chasing money for gambling has been one of the reasons most of these people have turned addicts from gambling but if they have played rather for fun maybe they wouldn't have gotten to the point of addiction and as you rightly stated Its mostly common amongst developing countries where the citizens in the bid to make more money will continually indulge in gambling activities with the hope to some day be lucky and win a good amount of money that will transform their lives for the good.