It is so difficult to listen to absolutely nothing. 

But it's not too hard to tune out everything.

That's true. Daydreaming is easy. 

Encouragement of self-reflection:

Without stimulation and distraction, your brain need not focus and goes into a default mode of sorts. That doesn't mean it completely turns off. Quite the opposite. Your brain at rest will sort and gather information. This is where the self-reflection comes in.  

Auditory stimulation forces your brain to process sound and listen to what's going on around you. Without that external noise, your brain is forced to listen to what's going on inside of it.