What's a real life usage for detecting communities in complex systems? I am reading through the Girvan-Newman description, having come from the topic of "detecting communities in a complex system".

What exactly is a community in this context and why is it useful for a network to know about it? In simpler terms, what's a real-life usage of this algorithm?
Suppose you have a bunch of friends, regularly meeting together. Occasionally, you also visit your sibling, and they have their own friends. Can we extract these groups of friends just from knowing who's friends with whom on a social network? This is community detection, which is a form of intelligence gathering.