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

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

But in the example:

[QUOTE]

<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.

[/QUOTE]

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.?

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