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

Food Street - Restaurant Management & Food Game Food Street - Restaurant Management & Food Game
"Events"

Minecraft Minecraft
"Some achievements come with optional skins for your player character."

Animal Crossing: Pocket Camp Animal Crossing: Pocket Camp
"Some events that you need to do repetitive tasks to complete have a fortune cookie as their reward, which gives you a random item (the collection of which depends on the cookie, that much you know from its name and appearance.)"

Baseball Superstars 2020 Baseball Superstars 2020
"you can make your own quests which can give you tickets to summon random trainers."

WitchSpring2 WitchSpring2
"some random material grinding but not that hard."

WitchSpring3 WitchSpring3
"basic grinding for materials not too much though."

WitchSpring WitchSpring
"grinding materials but they are very easy to obtain."

Stealth Master Stealth Master
"Open a "Chest" after a certain objective is reached. Said chests claim to have a percentage to give you a new character to play as. These claims are probably misleading."

Doodle Jump Doodle Jump
"Random challenges in exchange for a random amount of a random ingame currency"

Vector for iPhone Vector for iPhone
"sometimes coins can be golden or Ruby in a level making them worth double or triple of a regular coin"