Guild Wars 2

Why do level 80 characters do the level up animation when the can no longer level up?

you still keep getting skill points for each “level up” after level 80

When you hit level 81 you gain another skill point and get de-leveled back to level 80. These extra skill points are used to create Legendary weapons and to buy Mystic Forge items such as Eldridge Scrolls. Therefore the level up animation after level 80 is still useful.