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
War and Order"Limited to upgrading one building at a time. Paying opens extra upgrading slot. Troop and hospital sizes locked behind building levels."
Tower of Fantasy"500 item inventory limit for all items. Players constantly have to break down useless equipment or use food."
IdleOn - Idle MMO"Storage maximums"
Love Live!School idol festival"You can only have a set amount of space for your cards. Irrelevant if you just play songs, but a pain for collectors. It technically takes premium currency, so it's a YES, but it's so little you don't actually have to pay."
The Battle Cats"You have to spend Catfood (premium currency) to increase the amount of available unit lineups."
The Wanderer"It does have a capped inventory system, however you can expand that through the in-game currency. If you run out of in-game currency, you can choose to buy more."
Merge Studio"Yes it sure does! And it’s quite expensive the more inventory you buy the more expensive it gets"
Travel Town"You need to spend the in-game currency of "gems" to increase your inventory. You can earn gems slowly without spending real money, so I have gradually increased my inventory for free over time, but it tries to encourage payment"
The Ssum : Forbidden Lab"You can only keep a certain amount of emotions in your inventory, and you can't collect more of the same emotion until you spend what you have. but you can only visit another planet if you have the incubator going. which you can't do if you can't collect the emotions already done incubating without having to pay for another vial or whatever."
MARVEL Contest of Champions"Summoners sigil subscription expands inventory size"