Some people frequently using an HDMI cable to use their Mac with an HDTV. Usually after plugging into the HDMI cable they have to manually switch their Mac's sound output to HDMI in order to send the sound to their TV as well. The priority device list allows Sound Control to automate this behavior.
By turning on "Switch to priority device when attached" and arranging your audio output devices in order Sound Control can now manage the default output device for you.
For example, if we have HDMI at the top, followed by Internal Speakers. Now, when we plug an HDMI cable into our Mac the sound output device will automatically be switched to HDMI. When the HDMI cable is unplugged it will go back to Internal Speakers.