From a comment on Mike Norman’s blog.
This helps to ensure a wide output gap.

From Bernanke’s testimony from a couple of weeks ago:

“Having a large and increasing level of government debt relative to national income runs the risk of serious economic consequences. Over the longer term, the current trajectory of federal debt threatens to crowd out private capital formation and thus reduce productivity growth. To the extent that increasing debt is financed by borrowing from abroad, a growing share of our future income would be devoted to interest payments on foreign-held federal debt. High levels of debt also impair the ability of policymakers to respond effectively to future economic shocks and other adverse events.

Even the prospect of unsustainable deficits has costs, including an increased possibility of a sudden fiscal crisis. As we have seen in a number of countries recently, interest rates can soar quickly if investors lose confidence in the ability of a government to manage its fiscal policy. Although historical experience and economic theory do not indicate the exact threshold at which the perceived risks associated with the U.S. public debt would increase markedly, we can be sure that, without corrective action, our fiscal trajectory will move the nation ever closer to that point.”

Full prepared remarks