In polymorphism what is the practical use of using a parent class reference variable to point to child class object instance?

Consider following scenario...

I want to develop the new IDE.
I will provide support for various databases like mySql, oracle etc.

Now, I will develop one interface

Interface MyInterface
{
  public int Insert();
  public List<object> Select(); 
}

I will give this Interface to DB providers for uniform implementation.

DB providers wouldn't allow me to see their implementation of those methods.

So to access those Methods what I need is to create the MyInterface's Reference and I will assign the object reference of the classes of DB provider.
This way I can access the methods of DB providers without seeing their internal implementation of code.

MySql DBProvider...

class MySqlDBProvider : : MyInterface
{
    public override int Insert()
    {
       //here their Insert logic goes
    }
    public override List<Object> Select()
    {
       //here their Select logic goes
    }
}

Oracle DBProvider...

class OracleDBProvider : MyInterface
{
    public override int Insert()
    {
       //here their Insert logic goes
    }
    public override List<Object> Select()
    {
       //here their Select logic goes
    }
}

Now what I need is...

using MyInterfaceNamespace;
using MySqlDBProviderNamespace;
public class MyClass
{
   //here i will create reference of MyInterface
   //and assign Object of Your DB Provider Class.
   MyInterface MyIObj = new MySqlDBProvider();
   MyIObj.Insert();
   MyIObj.Select();

   //I can do the same for Oracle
}

Hope You Understand the importance of this feature in this kind of scenario.

Hope This Help
-----------------
Pratik Bhuva