If you work in networking sooner or later there will be a situation where someone will bring you a device that you must connect to your network that was used somewhere else before. That can be something like switch, Fiber channel switch, some security appliance, different controllers etc. In that case this device will often have a static IP address assigned and you will not know whish address that is. In that case you are unable to communicate with the device and if you are connecting it to your network he will have different IP from you subnet. That will make the device unreachable.
You need to find this unknown IP address in some way so you can connect to the device and then change the IP.