Why Polymorphism ?!

Question

why we need 'Polymorphism' , what is our need to it ??
for example : we have 2 classes called C , D inherit from class A , each of them contains d() method that prints a diferent text . and we want to print its content after objects definition for the subclasses(C, D)
this the way :
C c= new C();
D d= new D();
c.d();
d.d();
this the usual way we learnt but with Polymorphism :
A a1= new C();
A a2= new D();
a1.d();
a2.d();
isnt the first way of defintion is enough ?
this is more diferent and complex . that makes my mind frozen . In fact this section is so hard to understand
i have many question about Poly.. but the only question makes me confuse me is why Polymorphism ?

Answer 1

When done right, your main processing code only deals with class A. It never needs know C or D or some future class exists. Therefore, adding E to the system based on A doesn't need as many changes. You create E and it is done running as if it was there from day one.