Waste Aversion

Capped inventory forces you to destroy items or upgrade inventory. Also, having small amounts of left over premium currency.

People to do not like wasting money, time or effort. Games are sometimes designed to force you to waste these things unless you pay to prevent it from happening.

One way that games do this is with a capped inventory system. If you are only able to hold 20 items and you collect another item, you are forced to discard or destroy the extra item. Games that do this will usually give you the option to pay to increase your inventory cap, and thus allow you to collect more items before your inventory is full again. By making it difficult or annoying to manage your inventory, games incentivize players to upgrade their inventory cap to push off this inconvenient task into the future.

When premium currency can be purchased in different amounts than what you can spend, you will always have some small amount left over. For example, if you can purchase gems in multiples of 1000 but only spend them in multiples of 80, you will have 40 gems left over at the end. Abandoning the game with unspent premium currency feels like wasting money, so players keep playing and keep purchasing more premium currency with the hopes of getting down to zero eventually and having nothing wasted.


Examples

Girls' Frontline Girls' Frontline
"Almost everything can be expanded and usually requires gems to do so, of which a limited amount can be freely obtained over time."

Old School RuneScape Old School RuneScape
"Buying a membership doubles the size of the player's bank from 400 stacks of items to 800 stacks of items. There are also members' items that allow for more efficiently carrying things in the inventory."

Candy Crush Saga Candy Crush Saga
""Piggy bank" system. Your piggy bank fills slowly every stage you win, and you have to pay to crack it open to get the premium currency you've "collected" over the stages."

Pocket Frogs Pocket Frogs
"A one-time purchase that drastically expands inventory and mailbox"

Ragnarok M: Eternal Love(ROM) Ragnarok M: Eternal Love(ROM)
"Premium purchases allows to expand the inventory (temporarily or permanently)"

Dream Girlfriend Dream Girlfriend
"Somewhat, you can buy extra storage but over months it becomes less of a problem."

Mario Kart Tour Mario Kart Tour
"Gold Pass subscribers can earn 600 coins a day where normal players can collect 300. They can also earn more XP per day, which can level up karts, characters and gliders, which then give more points when used."

Harry Potter: Wizards Unite Harry Potter: Wizards Unite
"You can only hold a certain amount of potions, ingredients and energy. You have to spend gold to increase the limits."

Star Trek Fleet Command Star Trek Fleet Command
"Protected resources are dependent on levels of player advancement and researching. This same thing applies to ships."

AFK Arena AFK Arena
"Although it never has felt like there was a need to increase the inventory size at any point in this game."