Collars are often used in a BDSM context and some people use it for random play, while others are looking at it for more serious reasons. To me its a symbol of the relationship that exists between the Dom and sub, so I guess that I use collars for a more serious reason.
To me it is important that the sub wear the collar as often as possible but that also comes with a problem. A lot of collars are quite revealing in their design – They definitely look BDSM with the leather and rivets as the most common design. This makes it hard to use a collar in an everyday situation without looking like a punk-rocker. The leather and rivets collar is of course very usable for humiliation play or public play as the sub has to hide the collar in order to not be revealed and embarrassed.
But there are other collars, pretty enough to be worn at all times as they are more designed as a jewelry with a contemporary design. Dane Gundersen provided me with a very nice link to a company that, in my opinion, does very beautiful collars.