This.Not much.
1) Tradition.
2) A way to quickly describe a character's personality. Just as class describes a character's abilities and level describes power. 1st level LN rogue. 15th level CE barbarian. That short string tells you a surprisingly large amount about an individual.
Is there even any other rpg system out there that has alignment?
I felt it was an odd system when I first came into contact with AD&D (1E). While I have grown to accept it, I've never like it a lot.
I prefer systems like Ars Magica's personality traits that allow for a wider and more complex range of characters while retaining a way to integrate them into game mechanics.