Due to its black color, the Coat of Midnight boosts the «Hiding» skill of a player.
Kirito acquired the Coat of Midnight as the Last Attack bonus drop from the 1st Floor boss. After the defeat of the boss, some of the raid party's members brought up the issue of the Beta Testers having exclusive information about the boss, which was believed to have led to the death of the raid party's leader, Diavel. Fearing that the discrimination of Beta Testers would never end, Kirito proclaimed that only a small number of Beta Testers actually had more experience and information than regular players and dubbed this small group of players as Beaters to separate himself and others like him from all other Beta Testers. He then donned the coat in front of the raid party to show off his status as a Beater, thus taking all the hatred towards Beta Testers onto himself.
After this theatrical performance, he only used the coat when he was outside the city, as he would take it off before going into town to hide his identity, since he had become Sword Art Online's most despised player.