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

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"

Make More! Make More!
"The event gives you random items depending on how well you performed. Ad boosts also give you random power-ups."

MARVEL Future Fight MARVEL Future Fight
"'Bonus Missions'"

Legends of Runeterra Legends of Runeterra
"Random loot from loot boxes with a random chance of an upgrade."

Castle Cats:  Idle Hero RPG Castle Cats: Idle Hero RPG
"Mostly the rewards for going on a quest are stated, but there can be other things thrown in."

SCRABBLE SCRABBLE
"All chest rewards include random parts of tiles, arena tickets, and/or gens."

Infinite Lagrange Infinite Lagrange
"XP for ships on successful attacks"