I was always surprised when working for an ISP clients would ring and ask why they have gone over their quota. These aren’t home users many of these were major corporations spanning several sites with equipment several of our datacentres. Even in small networks it is important to know what data is traversing your network even if it is just to know who is running bit torrent and slowing everyone’s internet down. But it also has many more uses then that. It is a great way to detect anomalies in your network which most often are caused by Virus/Malware/Trogens all the fun stuff.