If you want to fly, then you needn't know anything except what frequency you set your radio to.

If you want to push the limits and expand the envelope, you must have good aerodynamic knowledge and structural knowledge of your plane or else you:
a) won't know where to begin
b) won't know how to recover from unusual attitudes
c) will overstress the airframe and yourself
d) kill yourself and or wreck the plane.

Simple as that.