It's an anthropological thing. Identifying with your own kind ; exhibiting signs of group membership; feeling safe and protected (and accepted) within the "herd" ; reinforcing bonds and links with others of a similar bent (no,I don't mean that ).
               Apart from that,it satisfies a need in all (or most) of us ,to throw off the shackles of sartorial convention and be "ourselves" for just a little while.Or,like Monsieur Le Rouge,all the time, I guess !!