I have been so confusing about the w component for ages, until I read this:
Explaining Homogeneous Coordinates & Projective Geometry

So I quickly made a simple to shader to test it out:

To summarize, w component is the

The W dimension is the distance from the projector to the screen(object).

when w > 1, the object looks far (smaller)
when w = 1, the size remains the same
when w = 0, it’s actually covering the whole screen

This is the reason why we have make sure the w component is correct when we are doing _Object2World and _World2Object

Also a interesting point to note from the blog post:

If W=1, then it is a point light. If W=0, then it is a directional light.

(Thanks Kemal for the tutorial links!)

The shader is a mixture of

hull and domain (tessellation)
https://gist.github.com/JohnnyBeluga/9bd5c13a2b37706852a5
+
geometry (extra polygons)
http://ellenack.com/2016/11/05/grass-geometry-shader/
+
geometry (wireframe)
https://unitycoder.com/blog/2016/11/28/wireframe-shader/