Its an option for flavour.
Why bother having a green fire quest for warlocks if its the same as red?
Why bother having different haircuts if you are just gunna wear a helmet anyway?
Why bother letting me pick my gender on a blood elf if they both look female anyway?
A real life example, this is like complaining that the side you were on lost a war, 'Why let me enlist if you were just gunna lose?'