Wearing dark glasses or mirrorshades is long-standing visual shorthand for Badassitude. The act of putting them on means that it's time to kick ass, take names or do both. The act of taking them off is a sign that somebody's about to get scrutinised by the badass, usually just prior to an asskicking.
Obligatory for a Cyber Punk setting, or for intimidating Deep South Lawmen. A nice accessory to a [...] [tvtropes.org]