Although generally worn by males while dressing up, it is slowly becoming acceptable to wear casually.
When girls where ties, they are subjected to torture of trying to be "unique" or an Avril Lavigne poser. Neither of which is necessarily true, some wear them just because it sounds fun (they do make great accesories!).
2. also helps dominatrixes identify potential clients who are passives.
(Origin: when slave-catchers would tie a rope around a slave's neck. Also, symbolizes iron collar and chain used to restrain slaves).