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

Dragalia Lost Dragalia Lost
"Dragon storage."

Pokémon Café Mix Pokémon Café Mix
"In the Remix update, staff levels are capped unless Acorns are used to unlock them."

Genshin Impact Genshin Impact
"Premium currency can end up at values below any of the items in the shop, possibly incentivising the player to buy more."

Castle Cats:  Idle Hero RPG Castle Cats: Idle Hero RPG
"You can only have so many cats out on a quest. You have to increase your storage to have more cats and send more cats on quests."

Pokémon GO Pokémon GO
"IT HAS TWO - both for mons and items"

AdVenture Capitalist AdVenture Capitalist
"You pay real money for Gold but the gold is in weird bundles of 55/115/240/624/1300. But most items in the shop are multiples of 10 aside from one item which is 25 gold - eg if you bought the 55 gold bundle, you would always have 5 gold leftover unless you used the gold 'inefficiently' (eg. buying 3 1d time warps 10g each and one 7d at 25g, which gets you less overall time warp days than 2x 7d time warps at 50g total with 5g left)."

Granny's House: Horror escapes Granny's House: Horror escapes
"Some things you unlock with the special subscription or some sort of upgrade you get."

Flower Garden - Grow Flowers and Send Bouquets Flower Garden - Grow Flowers and Send Bouquets
"Flower pots, which can be unlocked by either using credits or paying money."

Legend of the phoenix Legend of the phoenix
"You need to pay to - open seats in the Academy - open more pigeons - open more slots for Foster children - open more plots of land for farming"

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."