The purpose of an interpersonal communication may be informal while that of a business communication is strictly formal and the audience is a business acquaintance of yours so the tone has to be mature, serious and businesslike as you are talking about important work. The content during the communication is different because the things you talk about in the two cases are entirely different. For example, if you're talking to a friend or personal acquaintance of yours, your tone and words would be different from what it would be when you're communicating with a business acquaintance about getting something done or any other thing.
