Dd Eventlisenter For Mouse Click To A Movieclip?

from various books, I know that to add eventlisenter for mouse click to a movieclip:
 
movieClipName.addEventListener (MouseEvent.CLICK, clickFunction);
 
then I define the function
 
function clickFunction(event:MouseEvent)
{ bla bla bla }

My questions are (as silly as they might sound!),

a) why do we need a parameter (i.e. event:MouseEvent) for clickFunction??

b) since we have a parameter for clickFunction, when we feed this function into addEventListener, why didn't we write  movieClipName.addEventListener (MouseEvent.CLICK, clickFunction() );