Doug Elmendorf, director of the politically neutral Congressional Budget Office, explains it:
Although there are tradeoffs in deciding how quickly to implement policy changes that would reduce future budget deficits, there are important benefits to deciding quickly what specific combination of reductions in spending and increases in taxes will be used to put fiscal policy on a sustainable path. Most important, enacting policy changes soon would allow for gradual implementation of those changes while still limiting further increases in federal debt and the negative consequences that would flow from those increases. Moreover, enacting policy changes soon would probably provide some boost to economic activity by reducing uncertainty and holding down interest rates.
(Hat tip: Adam Sorenstein)