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.
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
Drilla: Idle Gold Miner Game"This game forces you to sell ALL of your ores of one type, but with a few in app purchases you can then partially sell. These ores are also used for upgrades in the game."
Yu-Gi-Oh! Master Duel"You can only have 10.000 free gems in your account balance at any given time which amount to 100 Packs. Any more gems you get will get send to the Gift Box which have expiration dates on when those items will be deleted. This mean that you can only save your gems at most for 1 months."
PUBG MOBILE"wardrobe option that allows you to swap clothing set-ups can be bought for UC (in-game currency)"
Invasion: Modern Empire"ghost camp capacity can be increased by spending blood diamonds"
Grim Quest - Old School RPG"inventory can be expanded with ingame currency, but doing so would severely handicap character progression effectively forcing to use in-app purchases of game currency"
Bud Farm: Grass Roots"to increase you need to build a new version using materials and coins. if not enough - pay"
FINAL FANTASY BRAVE EXVIUS"Energy is capped, inventory is capped, unit limit is capped."
Ping Pong Fury"In the game, you'll encounter lootboxes called 'pods' that are essential for progression, with a maximum limit of four pods in your inventory"
Phobies"There is a limit on the amount of tears, experience and beans you can earn each day. You can increase the daily limit via a subscription service or by in game currency. If bought using the in game currency, there is a time limit which when expires, you have to buy again"
A3! Actor Training Game"There's an initial 200-card cap"