The preview is an on/off to preview the audio being sent to SWB Audio Capture (the actual audio driver), to some device.
Want to know more about how the SWB Audio Capture driver works? see: https://youtu.be/CQUTadBGnDA
Device to "Preview to"
The dropdown lets you choose the device you want audio to be played to.
When you enable preview, the red checkbox above goes green, and any audio seen on the input side of SWB Audio Capture is played to the chosen device.
- It can be 'Default Output'. This means, the same as 'system output' in preferences.
- It can be a specific device. If you use this option, audio is always played to this device regardless of your setting in System preferences | Sound.
Note: The audio driver always works (pipes audio), regardless of this option. This is just a helpful preview. It doesn't affect the operation of the driver.
When to use it?
You don't always need it, it is application dependent.
When to use:
- When the app your capturing cannot monitor audio (e.g: zoom. discord. garageband)
When you don't need to use it:
- When the app your capturing has its own monitoring capability. e.g: OBS. When you've a more complicated setup (as OBS setups tend to be), then you probably want to keep all the settings for audio in one place. Typically that's going to be the app managing those settings / monitoring.