Why we assign the reference of derived class object to the base class reference in java?

Your example is not so good.

Let's try something different: Vehicles. You can assume that all Vehicles have an engine with a certain power:

public class Vehicle{

   private int iPower = 0;

   public Vehicle(int iPower){
       this.iPower = iPower;
   }

}

Now, there are different Vehicles on the road. e.g. some trucks, some cars and some tractors. All those Vehicles have an engine - but some might have additional features:

public class Truck extends Vehicle{

   private int iLoad = 0; // in tons

   public Truck(int iPower){
      super(iPower);
   }

   public Truck (int iPower, int iLoad){
      this(iPower); // calling 1 arg constructor
      this.iLoad = iLoad;
   }
}

public class Car extends Vehicle{

   private int iSeats = 1; // minimum

   public Car(int iPower){
      super(iPower);
   }

   public Car(int iPower, int iSeats){
     this(iPower);
     this.iSeats = iSeats;
   }
}

We have Vehicles as a base class, Trucks and Cars as more specified Vehicles.

Now here is the trick:

We will set up a Road and let Vehicles - no matter what kind of Vehicles those are - drive on it:

public class Road{

   public void drive(List<vehicles> oVehicles){
   // funny traffic action
   }

}</vehicles>

So the base class is a general approach to work with "the group" of those objects. We can cast the Vehicle to the more specified version if needed:

Truck oTruck = (Truck)oVehicles.get(0);

But that is just needed when we want to do special things. In general these are Vehicles - that's enough to let them use the road.

This system can be used in a lot of places:
A natural person e.g. is defined by gender, name and date of birth(or) place of birth.
A customer is a natural with a history of bought items.
An employee is also a natural person having other properties (right to enter the depot, can take cash).
A robber is a natural person with a gun.

But all of those natural persons are walking through the shop. Until...