This is a fairly widespread and varied fetish, and most people don't like associating it with being a fetish. However, as a fetish is generally considered to be something that becomes an object of extreme devotion, the uniform meets the criteria. Since an individual becomes more enticing because of the uniform they are wearing, the uniform is therefore an object of some form of devotion.
The uniform in question can be many things. The cheerleader, nurse and Catholic schoolgirl uniforms are popular with certain individuals. The military, police and fireman uniforms are popular with other individuals. The school lunch lady uniform is popular with a very limited number of individuals. Most of them belong in cages.
Why does the uniform create an added dimension to the sexual fervor? It may be associated with something in the memory or imagination of the individual in question. Boy was shy in high school and never part of the popular group. Cheerleaders were in with the popular group and were considered untouchable godesses in some way. Manly fireman rescued favorite kitty cat from tree when person was ten years old. He stayed in the memory and was remembered a heroic and manly figure.
Most fetishes seem to be brought on by experiences early in life. Others seem to be inate and inexplicable. The uniform may simply fulfill some kind of fantasy. The police uniform may invoke an impression of power and therefore satisfy a desire to be dominated or punished. The nurse uniform may invoke a scenario of compassion and care along with helplessness in the hands of said nurse. Whether there are fantasies that revolve around the uniform due to the way they stimulate the sexual imagination or because they bring to light experiences from the past, the uniform has a sense of power. It means something to the uniform fetishist. It means something they cannot deny to themselves, even as they may hide it from prying eyes. So, don't be alarmed when your partner asks you to put on some form of uniform or costume. It is still you they are making love to. The uniform is merely symbolic, and fantasy is good. It is very good indeed.
Certain users have asked me not to forget their own relevant uniform fetishes. Seems we have fans of maid uniforms and ninja uniforms out there. Probably quite a few others as well. Also, Footprints would like to note that "fantasizing about sex with nuns is a habit I've gotten into." Ehem. Well at least it isn't school lunch ladies. This was never mean to be a comprehensive listing. Such a thing would end life as we know it on this planet.