The answer lies in Object Oriented concept. When you are
overriding a method, you overriding the internal details of
default behavior.  But your interface to external world is
not changed. Example, Base class employee has a method
commuteToWork(). All the manager in the company has given
the company car and a driver. Their implementation of
commuteToWork will be different from the non manager
employees.  If you add a new method saying
commuteToWorkByCar(), every Monday morning you would need to
call different method for each subtype of employee.  Hope it
will help.

hi
   MethodOverriding is the concept which implements Dynamic 
Code(Method) binding.
   i.e., The method which is going to bind or execute is 
decided at the runtime basing on the Object that we are 
using to call that method...
 Suppose assume we have two Classes A,B Where class B is 
inheriting class A.And even we have overridden(given 
different implementation with same method signature)a 
method of class A in class B.
   Now the criteria is which method(superclass method or 
subclsas overridden method) is to be called or executed is 
determined based on the object(class A object Or class B 
object) we are using to call that method............

Bcoz......wen we think in Object Oriented Lang ..has u ask 
if we create a new method we hav to call the method each 
time wen we invoke the Object & it is a performance ISSUE 
it wil eat lot of memory & it is a COMPLEXITY 
Issue  ........if u override Using the superclass Handle we 
can create  any instance which is of any type(int Float 
Long Short)it wil save space and reduce COMPLEXITY and 
increase PERFORMANCE...