U.S. will reach debt ceiling on Dec. 31: Geithner

[julian09]

6. Congress authorized spending with insufficient funds.

They have power of purse and have obligation to authorize debt to continue payment for the money they already spent.
This is unnecessary step because they already authorized the expenditure by passing the bill in first place.
They are using the debt ceiling to extract concessions or more accurately hold hostage the country's credit rating, for political gain.