What is port mirroring and in which cases it can be used and benefits of using this ?

Question

One server may have support of limited number of physical interfaces. What if there is need of more Ethernet interfaces but physical slots are not available ?

Answer 1

Port mirroring is a concept by which packets exchange between switch ports are sent to one specific port for the further analysis.
It is very useful but you configured a switch port with a particular configuration and want to check whether packets going out from that switch port is proper of not. Assume a switch port is configured with VLAN tagging and you want to check whether packets are going out with tagged VLAN or not.
Another scenario if a hacker gets control of a switch he can get all the packets and get some sensitive information in case user is using http (un-secure) protocol for browsing.