What is the purpose of reversing face normal of a polygon ?
I couldn’t seem to find the answer through google 
To have the normal on the opposite side?
One example is a simple cube.
If you are look from outside the cube the normal have to look out so that you can see the cube.
If you are inside a box you probably like to see the walls of the box, so you have to reverse the normals so that they point inside.
The polygon is only viable if the normal is pointing at the camera direction.
So, by default “back face culling” is enabled.
Which means that any polygon which the normal points away from is not rendered.
Okay, say you were making a game that took place inside a sphere.
What would happen is that when inside that sphere you would see nothing ( because the normals of the sphere face away from you and so are culled ).
If you reverse all the polygon normals of that sphere so they point inwards, now when inside the sphere you will now see the walls.