If Heather is sitting at her computer, and wants to invite Chris to lunch, she could send email to him, but he may not get the message in time. If she has a talk program, she can invoke it and tell it to notify him she is calling. If he is online, he will get a message that Heather is trying to reach him. If he chooses to make the connection, they will both get a screen on which they can type simultaneously. They can interact directly and determine where they will meet.
Chat programs illustrated their relevance during several recent world emergencies. Notably, during the war in Chechneya there were few formal communication channels open. There were, however, people sending messages worldwide on chat lines. Similar communication occurred during the Russian coups attempt and other major crises. Less dramatic uses of the technology have been used to have on-line conversations about sporting events and political debates as they occurred, and simply to waste long evenings. There have been a number of romances over chat groups, and at least one wedding which occurred on a chat line.