Promiscuous mode definition
A network card configuration that passes all traffic received to the network adapter driver and protocol stack. Network analyzers, protocol analyzers, and packet sniffers use promiscuous mode to investigate network traffic and detect potential vulnerabilities.
Real promiscuous mode use cases
XLink Kai is a computer program that reroutes traffic from older consoles on the same Wi-Fi network to let you play their games online.)
Wireshark is a free open-source packet analyzer. It uses promiscuous mode and other techniques to capture all network traffic for study.