Unpredictable or random rewards are more addictive than a predictable schedule.
"There are small rewards for everything from checking the news page to sharing a screenshot"
"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"