Capped inventory forces you to destroy items or upgrade inventory. Also, having small amounts of left over premium currency.
"In the Remix update, staff levels are capped unless Acorns are used to unlock them."
"Premium currency can end up at values below any of the items in the shop, possibly incentivising the player to buy more."
"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."
"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)."
"Some things you unlock with the special subscription or some sort of upgrade you get."
"Flower pots, which can be unlocked by either using credits or paying money."
"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"