It's only for python getter at initialization ?

yes, it is.

This is a dupli code, can you write a function like "SetBackroundFromWorldColor" ? which take as argument the scene or the world info.

float *KX_WorldInfo::getBackColor(void)

I think Blender style is float *getBackColor();
Also I think this method can be private.

How you want to write this methode as private ? it used in ImageRender, if you want safety the best is :
const float *getBackColor() const;

Sorry my fault. I write the comment to the wrong method.

This method can be private.

Maybe, I can reuse the SetBackGround(KX_WorldInfo* wi); that it is not used?

I think SetBackGround() and SetWorldSettings() are relict's, and should be delete with the next cleanup.
Basically it make no difference if you declare the method private, protected, private. It is only god for non experienced programmers and it reduce the amount of code completion suggestions.
You can use the old method, you can make your method protected or you can leave as it is, if you prefer this. From my side it was only a suggestion that this method don't need to be public and can be private because it is not used anywhere else.
I personally have no strong preference about this.