fed to begin quantitative easing today, page-16

I'll underscore my point with the following statistic taken from here:

http://www.cnbc.com/id/29880401/?slide=16

That’s right, the biggest holder of US government debt is the United States itself. The Federal Reserve system of banks and other US intragovernmental holdings account for a stunning $4.806 trillion in US Treasury debt. And with recent announcments from the Fed, potentially another $1 trillion may be added to its balance sheet... About a decade ago, the total government holdings were "only" $2.5 trillion.

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In the context of that - would someone tell me why 300 billion worth is supposed to be so significant?