Sometimes there is just no substitute for looking at raw packet capture data when infiltrating troubleshooting a network. In normal situations, I'm content with loading Wireshark or Network Monitor up on a system that sits in-line with the data I require (e.g. sniffing packets from a DHCP server when you have clients that cannot get an IP address). However, there are many times when you cannot load sniffing software on the device in question, or don't want to. Perhaps you want to monitor inbound traffic to your ISP to a border router, or watch for packets leaving a device you don't manage. In these situations, you need a passive network tap to capture the traffic without touching the two end points to be monitored.