Flyby as an attack is best for large creatures that can attack a foe without entering the spaces that the foe threatens. For instance, a huge dragon may get a lot of benefit out of flying in and attack a medium sized foe from three spaces away and then flying off. Giving the ability to small creatures with no spell-like or spell abilities is a bit of a waste ...
That being said, your familiar can do all sorts of interesting things during the middle of his movement. He could, for instance, move to a door, close it and then move away. This may not sound too exciting, but it can be very useful if you happen to be readying a fireball for when the enemy burst through the doorway and you don't want them to see it coming ... Basically, the ability allows him to go someplace, do something and then duck back into cover and minimize his risk.