Galactus is supposed to be an ancient ultra-powerful being, while all Silver Surfer has is a tiny fraction of Galactus' powers. Galactus has the ability to grant more or completely take away the powers that he had granted to Silver Surfer.
In the movie, Galactus has traveled all the way to Earth and is beginning to feed on it when Silver Surfer rebels. What could Silver Surfer possibly do to not only stop Galactus but also banish (possibly kill?) him away from the Earth?
The only reason that I can think of is that it was required by the script but I wonder if there is some explanation somewhere that I missed.