Assigning object reference to interface variable why?

Hi All,

Could anyone please provide any link or explaination of why we create interface reference variable and assign the object of the implemented class to that(Line No.17).Here is the sample i got from msdn.

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();
}
}

Thanks

Regards

Rajeev