Each different robe represents something different, in practise.
But taking it down to basic human instinct and suchlike - if you want people to listen to you and pay attention to you then you're gonna need to catch their eye and make yourself look important.
IMO the whole "each robe symbolises something special" is just a good cover for the Church saying "please note, these people have more power than you do and as such it is them directing the celebration. You'd better listen or else God will smite you."