Why gacha is the 'gotcha' tactic of MMO sales
Making money in free-to-play games isn't as simple as pursuing a single revenue stream. Often studios are looking at multiple approaches to coax players to part with their money in exchange for various goods and services. Now we all know that psychological manipulation is a key part of monetization, which is why it behooves you to read this Gamasutra article on a specific type of microtransaction moneymaking called "gacha."
Gacha is derived from Japanese vending machines that people would pay for a random toy that is part of a set. The idea is, in both the physical and video game space, that by convincing customers to repeatedly buy objects for a chance to complete a set, the customer will often end up purchasing many repeats (and thus buy the same thing more than once). Gacha can be implemented in many interesting ways in video games, turning the process into a game in and of itself (that costs real money to play).
Is this method evil or entertaining? The article says that it leans toward the latter: "Gacha is a powerful game design technique that allows developers to successfully monetize on F2P market. It's worth to remember that gacha may be designed in numerous ways that don't exploit human addictions to gambling but entertain and monetize in a synergy."