What is the role of Auto-smooth? I have noticed this: the "Auto-smooth" 



I can't figure out what is its' meaning. 
I understand that i can use it combing with the smooth shading. 
For example:

A) FLAT
 

B) SMOOTH


C) SMOOTH &amp; AUTOSMOOTH 30°



D) SMOOTH &amp; EDGE SPLIT 30°



It seems that Auto-smooth works like the Edge Split Modifier, with the huge difference that doesn't add vertex and edge loop. My mesh from B) to C) has the same amount of vertex while mesh from B) to D) doubles the number. My question is:

Why there is this Autosmooth?

Could be useful for create hard edges in my low poly (it doesn't increase the number of my vertex) and let me have nice and clean surface for a possible bake with my high poly?

Thank you to anyone will help me!
With Blender 2.80 the use of the Edge Split modifier is deprecated in favor of new custom normals.

Custom Normals allow precise tweaking of normals beyond what auto-smooth can do 

There's a lot more information in the Normals section of the recent documentation site. Although I'm not a big copy-n-paster, this is what it has to say about Auto-Smooth:


  The Auto Smooth option is a quick and easy way to combine smooth and faceted faces in the same object.


In addition, when you use custom normals:


  This has the side effect of enabling Auto Smooth, as that is necessary to use custom normals. Once you have custom normals, the angle threshold of the Auto Smooth behavior is disabled – all non-sharp-tagged edges will be considered as smooth, disregarding the angle between their faces.


Which brings us to the link to the documentation for Auto-Smooth:
https://docs.blender.org/manual/en/2.80/modeling/meshes/structure.html#auto-smooth