the main reason to use polymorphism is generalization, as 
you can see in case of overloading. say if i have add 
function which just add two intergers. and i have one more 
similar kind of functionality then in that case i will do 
the overloading as only my internal implemantation will 
vary.

second thing is consistency in code.

and in case of overriding main advantage is you can invoke 
child class function dynamically(means by creating the 
object of base class as normally we do). 

suppose one of my function expects my base class type 
object and i want functionality of overrided function then 
in that case i can simply write (parent obj = new child();)