Games used to be a single retail purchase and then you would own the game and be able to play it whenever you want, as much as you want. With the invention of microtransactions (in-app purchases) and downloadable content (DLCs), game developers have developed all sorts of tricks and techniques to get people to spend more on games than they would have otherwise.
Some games, typically massively multiplayer online (MMO) games, require a
monthly fee to play. This incentivizes players to play a lot to get their moneys worth and this fuels addiction when combined with other dark patterns.
Many modern games have
premium currency. This is where real money is exchanged for in-game currency. Players can then spend small amounts of this currency for in-game benefits. Maybe they can
pay to skip waiting for something to happen or maybe they can
purchase an item that gives a significant benefit. Some games may even have a
pay wall that prevents continued play of the game until a payment is made.
By making the exchange rate difficult to calculate, the true cost of items is difficult to determine, and by making sure that you always have some un-spent currency the game can tap into people's
waste aversion to get them to keep playing. By allowing people to spend unlimited amounts of money, the game allows and often encourages people to spend more than they should.
Over time, an item that you purchased in the game may
depreciate in value because of power-creep or deterioration and you may need to make additional purchases to remain competitive.
Games may use interface tricks to make you
accidentally spend money or nudge you towards more expensive purchases with by using a psychological technique called
anchoring or
artificial scarcity.
Gambling addiction is also a very powerful way that games get people to spend more money in the game.
Loot Boxes are one of the most common ways that this is implemented. You purchase a Loot Box and then get a random chance at a variable reward that may be quite valuable. This variable reward system can be quite addictive.