Variable Rewards

Unpredictable or random rewards are more addictive than a predictable schedule.

Casinos know that slot machines are more addictive if they pay out a random reward at unpredictable times. If people always won back 80% of their money on each pull of the lever, it wouldn't be an exciting game and wouldn't be addictive. It's the unpredictable and random nature of the rewards that keeps people playing with the hope for that rare big jackpot.

A constant or predictable schedule of rewards is not very exciting. This is why games often randomize the rewards that you receive for finishing a level or defeating an enemy. By making the rewards like a slot machine, they are trying to make the game more additive. They may also be trying to extend gameplay by making you grind longer to get what you want. If you need a certain item, but it only appears as a reward 5% of the time, people will keep playing over and over with the hope of getting that rare item. If the game has limited energy and forces you to wait to play then people may end up paying money to skip the wait.


Examples

Honkai Impact 3rd Honkai Impact 3rd
"Battlesuit fragments/souls may or may not be obtained when completing one of the few missions that they are obtained from."

ANOTHER EDEN ANOTHER EDEN
"Most notably, the rewards obtained from daily dungeons are variable. Sometimes the reward for completion can be desirable, other times they can be insignificant and disappointing."

Elder Sign: Omens Elder Sign: Omens
"Whenever the player has an investigator complete an adventure during gameplay, certain items (such as common, unique and spells) are chosen from a pool at random. The only information the player knows ahead of time are the type and quantity (two unique items for completion, for example) of these rewards."

Beatstar - Touch Your Music Beatstar - Touch Your Music
"Chest rewards are "cards" of a random category of song, which when enough are accrued will unlock a song from that category."

Fer.al Fer.al
"The daily quests give you random rewards."

Pokémon UNITE Pokémon UNITE
"The game has a gatcha (Energy Rewards) that requires playing matches to earn points to use the gatcha machine. Chances of winning a valuable reward are low."

Angry Birds Dream Blast Angry Birds Dream Blast
"it can give free hours of unlimited playing time."

Mech Arena: Robot Showdown Mech Arena: Robot Showdown
"Loot boxes offer random amount of premium coins and upgrade materials"

Flip Tumbling Flip Tumbling
"Spin wheel for rewards"

Love Live! All Stars Love Live! All Stars
"After-show ‘chest’ rewards"