The appeal is simple: Everybody has difference tastes when it comes to armour. What you think looks awesome, someone else might think looks absolutely terrible. At endgame, you are forced to wear the armour with the best stats. If you happen to think that the armour looks terrible then your experience is going to suffer. That then leads to reduced playtime and eventually you'll either roll an alt (hurting the endgame community) or quit the game (hurting the whole community). By adding a cosmetic system, the devs get around this issue and keep everyone happy. What they allow and disallow is purely arbitrary. I'd personally advocate letting people use whatever the hell they want for cosmetics because ultimately cosmetics have no bearing on gameplay, but I know other people get upset when they see a clothie wearing heavy armour, or a level 10 wearing endgame gear for example.