A timetable is a way to keep the Afghan government moving forward until they can take care of their own security. Taking away a timetable means no one is being held accountable for bringing American troops home. A definitive date puts more pressure on the Afghan government to get their stuff together in time for the troop withdrawal.
A timetabled withdrawal from Afghanistan is a promise of progress both to the people in Afghanistan as well as those in the United States. It is also a diplomatic move in publicly showing we have made calculated and planned improvements in Afghanistan. This timetable would ensure that progress made would not be lost overnight as could be the case if the withdrawal were more immediate.