the difference between viewbox width and height and the width and height in svg

[programAngel]

I read the following article:
http://webdesign.about.com/od/svg/a/...-attribute.htm

But in the example:

<svg width="800" height="400" viewBox="0 0 800 400">
This view box covers the entire page.

<svg width="800" height="400" viewBox="0 0 400 200">
This view box covers half the page starting in the upper right hand corner.

what is the point to set width and height in the viewBox that are half of the width and height that are set to the svg?

I still don't get the difference.