a) Feathers are lighter than plastic yet steer an arrow better. Specific feather sizes can be found here: Nitty Gritty.
b) Feathers grip the air and help “forgive” small release mistakes made under pressure.
c) Feather will increase the FOC or Forward Of Center balance point for your arrow. This makes your arrow MORE stable.
d) Feathers do not become overly stiff in cold temperatures or really soft in hot temperatures.
e) Feathers bend out of the way if your arrow contacts foreign objects in flight.