This practice was originally found among jazz musicians.In their case the reason was not fashion but practicality. One of the side effects of some drugs, notably amphetamines ( 'speed'), is that the pupils of the eyes are dilated and so let in far more light than normal.This can be very uncomfortable , particularly if the user is under a spotlight in a dark room. Use of amphetamines was , maybe still, common among performers because they stimulate, keeping the user awake and bright through long late hours. (Cannabis use may also cause sore, reddened eyes but it's amphetamines that were the main cause). Fans, whether users or not , simply imitated their heroes and the practice became a matter of fashion and image.