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

Gems of War - Match 3 RPG Gems of War - Match 3 RPG
"When you complete any puzzle, what you get from that puzzle is luck based."

RAID: Shadow Legends RAID: Shadow Legends
"Shard mechanics"

Fate/Grand Order (English) Fate/Grand Order (English)
"%chance of specific loot dropping for clearing different missions."

Walking Dead: Road to Survival Walking Dead: Road to Survival
"From end of stages or end of war matches."

Crossy Road Crossy Road
"Characters are rewarded through a 'capsule machine', which can provide duplicates."

Roblox Roblox
"As Rōblox is a platform of games (rather than a single game in itself), developers are allowed to do just that. If the title of the game you're playing ends with 'Simulator', you're definitely in one that does."

Game Dev Story Game Dev Story
"the way your characters make your games in game dev story is random"

Pokémon Rumble Rush Pokémon Rumble Rush
"Entire gameplay system revolves around random drops"

Pokémon GO Pokémon GO
"Research encounters"

Archero Archero
"Levelling up gives you a random talent level-up."