Some games contain forms of gambling. This is when you can spend real money to play a game of chance with the hope for a reward that is worth more than the money you put in. Gambling is a form of addiction, so this is a very powerful way to get people hooked on a game. In fact, it's so powerful that some governments around the world are putting laws in place to limit how much games can do this.
A Loot Box is a very common form of gambling that is found in modern games. Players purchase Loot Boxes using money or
premium currency and when they open the box they get a random chance at getting an item. Valuable items are less likely to appear than regular items, and the number and quality of the items can be
variable. Randomizing the rewards gives players an incentive to keep trying until they get the prize they want.
There are several psychological tricks that game developers can employ to make the gambling more powerful. The "Gambler's Fallacy" is a tendency for players to think that past events can affect the future probability of an event occurring. For example, if you have opened 10 Loot Boxes without success, you may mistakenly believe that the 11th Loot Box has a higher chance of breaking your dry spell and having something valuable inside of it. In reality, the odds are always the same and game developers may disguise this fact or suggest otherwise.
Another trick that is sometimes used is the "Hot-Hand Fallacy". This is the mistaken belief that a streak of luck will continue, when in fact the odds never change. The game may emphasize your successes and deemphasize your failures to make you feel like you are luckier than normal. This plays into a natural
Optimism Bias that most people have.
Some people are also susceptible to Apophenia, which is tendency to see patterns or relationships between unrelated things. For example, suppose a game allows you to purchase multiple Loot Boxes at once. Someone may mistakenly notice that they get better rewards when they purchase Loot Boxes in multiples of three. Maybe this held true for a few times, but in reality the odds are always the same.