The Allure of Spending: Mobile Games and Gashapon

"Gashapon" is used to refer to capsule toy vending machines that are popular in Japan. People are able to see which characters are featured in a machine, but won't know who they'll get until they put in money. Multiple mobile games from "Pocket Mortys" to "Puzzle & Dragons" use this system, where premium currency will offer a chance to obtain a rare and powerful character at the cost of getting an entirely different character. What is it about this system that gets players to spend their money once or multiple times? How many of top-earning mobile games use this system? Is there a "good way" or "bad way" to implement this system in a mobile game?