What is the purpose of MAV_CMD_NAV_GUIDED_ENABLE?

I get a COMMAND_ACK UNSUPPORTED back whenever I use it, and it seems unnecessary when I can just set the mode to GUIDED instead. I had a look at the Ardupilot source but it is not obvious to me what is going on here as I see references to MAV_CMD_NAV_GUIDED_ENABLE in mode_auto.cpp and AP_mission.cpp.

My best guess is does set mode GUIDED but only as a mission parameter to allow the user to take control at some particular part of the mission. Is this correct?