In this image, for example, I have this variable declared in a script.

public AudioMixerGroup routeToMixerGroup;

When I left click on that script I notice the field like in this image show up in an import settings. This is even before attached to any game objects.

So I think this is really great if you can specify a default connection to assets in your game. It is great for making a Unity plugin, and then you can make a “plugin settings” this way.

But when I AddComponent to the game object, the value I have connected seemed to do nothing. So what is the point of this slot in the import settings?

It’s for providing default references - for convenience.

So I think this is really great if you can specify a default connection to assets in your game.

Yep, that’s what it should do.

If you add a component to a game object that require references to something else, like prefabs to give a common example, you’d set up the fields in the importer. The references are stored in the meta file for the script. It’s similar to that you can set default values in the initializer for non-UnityEngine.Object serializable types, but the “how” differ:

public Color shirtColor = Color.red; // Defaults to red, for convenience
public GameObject shirtPrefab; // Uh oh, no simple way to make a default ref in script
                               // ... so lets set it up in the importer!

What you describe sound a bit strange to me - the connection should persist. Your IntroloopMixer should be an asset. If your IntroloopMixer is a reference to an object in a scene, then you can’t expect it to work because if you add the component to an object in a different scene, well, that IntroloopMixer wouldn’t exist.