The deal is intended to allow humanitarian aid to reach those most in need.
The U.S. and Russia have not had a positive history in cease-fire deals when its comes to Syria, and the U.S. is ultimately worried that trusting Russia with this new deal is a mistake. However, the difference between past attempts at deals and the present attempt is that the current attempt, while similarly trying to settle conflict-driven issues, revolves more primarily around people and the aid they need to survive the horror that has become their life, than the political conditions of the civil war.
The previous deal was set as a means of attempting to solve the civil-war crisis politically, whereas this deal, is less a matter of politics, but overwhelmingly a matter of shear human consideration.
Yet it is understandable why the U.S. is pensive about the current situation. There are potential consequences that must be weighed out, but the question is ultimately (that is, regarding this present deal specifically):
Will the U.S. take the chance of walking into a Russia/Assad ploy if it means possibly helping those trapped in Aleppo under unimaginable conditions (conditions that were exacerbated by the war which the U.S. is very much a large part of)?