Email is hardly real time. Telephone can be intrusive or expensive - for example one's mother might not want to call at 11pm at night but she can log onto msn and see you are at the computer and chat without calling and the phone waking everyone up. Also I talk to people in other countries and free real time chat is handy. It's very good for discussing things - many a time I have been on the phone to someone and talked about a web page or site and sent them the link via messenger. It's not a replacement but a complement. It's like saying why use email when post exists.