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
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"
Just Dance Now"The limit of 200 coins, which means you can only play a song a day, or 2 songs every 2 days. You can pay for VIP."
Baseball Superstars 2020"capped core system, trainer system, player system."
DISSIDIA FINAL FANTASY OPERA OMNIA"Not that bad"
Clash Royale"Limited lootbox slots that require payment to speed up in return for an open slot"
Destiny 2 Companion"With transmog yes"
Rec Room"outfit slots"
GODDESS OF VICTORY: NIKKE"Synchronization Device - The game has a synchronization device that equalizes the level of the characters in said "inventory," depending on five of the player's highest-leveled characters. The synchronization device requires 500 gems, albeit it not requiring real money, maximizing the number of slots available would require money at this point in time."