unity - Unity2d: orthographic camera and z-axis?

I wonder if anyone can help with an explanation of the z-axis and using unity with 2D with an orthographic camera.

Up until now i have only been modifying the x and y values of game objects, thinking this is the correct way. i thought the Z axis didn't exist in 2D space (i.e. unity 2d with orthographic camera) but I do notice that the orthographic camera is placed at -10 on the z axis, if i change this value then the camera disappears. I sort of began to accept that.

Now I cam to a point where I wanted to rotate something, and I notice that rotating eithe on the z or x axis, doesn't seem to do anything, only on the z-axis, there is that z-axis again !

Can anyone tell me when i should be worrying about the z axis in unity using 2D games.

It seems now that it is more important than I once thought :-)

Thanks in advance.