Ambient sounds are noises "in headspace": they have no location in the 3D scene and seem to be coming from everywhere and nowhere. You can attach non-positional audio nodes to another node in the 3D scene, but it is not a requirement. In contrast, you want other sounds to emanate from one particular spot.
The humming of the motor should emanate from the passing race car itself; hearing the steps of the guard approach from the right gives you the split second needed to dodge to the left; and the monster sneaking up on you in the dark is a lot scarier when you hear it breathe right behind your chair; hearing the ocean roar to your right and leaves rustling to your left helps you navigate the terrain.
3D sounds have a position in 3D space, therefore you must attach these positional audio nodes to (a node attached to) the root node. You need two things to render 3D audio:
An attached audio node representing the position or direction of the sound
The
AudioListener...