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

Need for Speed No Limits Need for Speed No Limits
"Must choose the correct card from the 3 after completing a race."

Head Ball 2 Head Ball 2
"Free loot boxes after leveling up."

Call of Duty®: Mobile Call of Duty®: Mobile
"Gives loot boxes for going up a rank."

Word Hop ‏‏‎‎‎‎ ‏‏‎‎‎‎ Word Hop ‏‏‎‎‎‎ ‏‏‎‎‎‎
"it can, i deleted it to be able to double check but for the time I played it the daily challenge fits here"

Hay Day Hay Day
"Derby rewards are randomized."

LEGO® Legacy: Heroes Unboxed LEGO® Legacy: Heroes Unboxed
"You are rewarded with various amounts of tiles or gear depending on how much you loot a level."

GrandChase GrandChase
"Rewards obtained after each adventure dungeon run"

Mobile Legends: Bang Bang Mobile Legends: Bang Bang
"Tiny amounts of Battle Points and experience for doing daily (and other time-limited) quests"

Candy Crush Saga Candy Crush Saga
"free boosters for some time-limited tasks"

Dream Girlfriend Dream Girlfriend
"Most dresses are behind gacha/lootboxes, with the rarest being less than 5% available per ticket. Even in paid options duplicates are a thing."