Because alignment is useful shorthand that gets you in the ballpark for a NPC/ PC, plus (from a DnD perspective) its central to the underlying metaphysics of the game (the outer planes literally correspond to various alignments).
I could assign personality traits to fiction protagonists. People have written whole theses and psychological reports on how Dennis from Always Sunny is a sociopath, or what personality type Tony Soprano is.
When I see 'Anakin Skywalker; Male Human Jedi 7, Ace Pilot 2, Jedi Knight 4, CG' I have some idea of a general ballpark for the Character (he's unconventional and doesn't play by the rules, but a nice guy).