Unpredictable or random rewards are more addictive than a predictable schedule.
"Some rewards aren't random, namely experience from trying new themes and from winning trophies. However, "Going out" randomly gives you Plot Points (an integral part of the gameplay loop), the player can randomly gain experience when working on a manga, and even training has an element of luck. On occasion, you may get a higher-than-usual stat buff from training your character."
"sometimes clearing story mode or event levels will randomly give you extra rewards no matter what your ending score is"
"coins to change aesthetics, but still makes you watch ads if you change"
"Almost every single reward in the game is random to some extent, however you do get the exact odds."