When apps use streaks or badges, they turn daily reading into a steady habit....

https://www.bookmark-maker.win/when-readers-see-a-streak-counter-they-get-a-helpful-nudge-to-return-tomorrow

When apps use streaks or badges, they turn daily reading into a steady habit. Seeing your progress bar move or hitting a seven-day milestone gives you a quick sense of accomplishment that keeps you coming back