I have tried top tube bags on a number of occasions for snacks and incidentals. I wind up hating them because as soon as I stand up to pedal, they wind up interfering with my legs. This bag holds more and has never gotten in the way. It's also quite aero tucked back behind the head tube.
I have the 2.4 liter version and in here I can keep several snacks, an Ottolock, and a Pearl Izumi rain jacket. There is also room for a small battery and a cable to go to a light or to charge my bike computer.
It's also very light and highly waterproof.