Because a costume part gains significance when is ab unlockable, and that makes you stand out as someone who shown commitment to, good example, helping the vanguard.
Not everyone role plays the same. Some role play in the confines of the game and when they help a faction, getting the costume peices means something because now they can use that item and without putting it in the bio, Everyone that sees them with the item automatically Knows what they achieved
Just like how some one in thier role play says that they got thier sword from doing blank, except in the confines of the game, I actually did was awarded the sword from helping the vanguard push back against the rikti.