Instanse of Class

Hi,

When i need create new instanse from Interfaces? and Why?

For example:

interface ISampleInterface
{
    void SampleMethod();
}
class ImplementationClass : ISampleInterface
{
    // Explicit interface member implementation:  
    void ISampleInterface.SampleMethod()
    {
        // Method implementation.
    }
    static void Main()
    {
        // Declare an interface instance.
        ISampleInterface obj = new ImplementationClass();
        // Call the member.
        obj.SampleMethod();
    }
}