Star levels apply special cosmetic animations to your cards for added prestige.
They do not affect any gameplay statistics such as damage hitpoints or speed.
Earning star levels requires donations or duplicates of a card past tournament cap.
It is a status symbol memorable during competitive ladder matches.
Supercell uses them to reward long term progress and show commitment.
They are popular among competitive clans and tournament players.
It is a way to flex without any gameplay advantage.