Example1: Most active studio monitoring speakers would use XLR connection to receive audio signal. This combo cable would be great to run the power line along with the "line" level signal to your studio monitor. (Please check if your equipment supports this feature).
Example2: your mixing board is located very close to the power socket. In this case you can run power to your pre-amp. or an amp., plus also run the XLR audio connection from your mixing console to a pre-amp. or an amp.