Port Mirroring (aka SPAN) is a functional solution for catching and observing network traffic in a non-intrusive manner. If you love to know its purpose, then this document helps.
Port mirroring is used on a network switch to send a copy of network packets seen on one switch port (or an entire VLAN) to a network monitoring connection on another switch port.
Intrusion detection system,
Troubleshooting using passive probe
Real user monitoring(RUM) technology that is used to support application performance management (APM)
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This is a hardware device which can passively capture traffic on a network, and is commonly used to monitor the network traffic between two points in the network.
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https://en.wikipedia.org/wiki/Port_mirroring
https://blog.niagaranetworks.com/blog/port-mirroring-for-network-monitoring-explained
https://en.wikipedia.org/wiki/Intrusion_detection_system
https://en.wikipedia.org/wiki/Real_user_monitoring