Why do informal groups exist in an organisation?

Answer 1

Because peope like to interact (chat) to others of similar backgrounds / educational experience / file goals ...

Answer 2

An organisation usually is an artificial set up and it works because people like to know there place within it's structure. Within that framework people will cluster in small conversational groups. Without these small informal groupings the organisation wouldn't actually function, because it enables people to relate to others and to feel a part of the whole