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
The World of Magic"Player inventory is capped by inventory bags, 4 bags slots can be crafted, 5 bags slots are extremely rare drops from enemies, 7 bags slots are gacha only on shop (they are tradeable)"
Flutter: Starlight"You can run out of space for collected moths, both in the main area and storage, it can be expanded for free but the costs get large and time consuming enough to just encourage using premium currency."
SOULS"Max heroes inventory that after a limit increases only with "VIP level". Rarely a problem though."
Idle Angels"You can expand your equipment storage capacity."
Super Snail"ok this one is a bit of a stretch but hear me out, there are a certain amount of relics that exist in game, new ones are added every so often in LARGE amounts and there are MAAANY relics of MAAANY stars so its not like you will probably ever hit that cap but there are certain P2W only relics so theoretically if there was a maxed out P2W player they would always out compete any F2P player altho this is basically impossible with new content added almost weekly"
Block Jam 3D"Maybe 70% of the coins that the player can get from completing a level gets put in a "piggy-bank" which needs around $8 of real money to be unlocked."
Pokémon Sleep"It is very possible to increase inventory size for free"
Isekai:Slow Life"only 1-2 examples of this. One is in the "trading post"."
Eternium"Character inventory and stash can be expanded using premium currency. Character slots are similarly locked, and even deleting an existing character to make room for another one requires premium currency!"
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."