"What is understood doesn't need to be discussed.". You don't need to tell me I don't recognize your name. 1). I will know that without being told. 2) it emphasizes in my mind that you are a stranger. ( stranger = danger ) 3) I may recognize it or for some reason at least think I do and think of you as an a acquaintance or at least a friend of an acquaintance and therefore give you a hearing.