Why define an object with an interface instead of a class that implements the interface ??

Interfaces are great for ensuring that classes are defined correctly, but I have seen code where an object  is defined according to the interface instead of the class EG :
imyInterface X = new myClass();  
instead of
myClass Y  = new myClass();  
where

myClass:imyInterface
{
......
}
In both definitions of objects “X “ & “Y” , then the object will be defined as a myClass.   The disadvantage of the definition for ‘X ’ is that in Visual Studio, clicking on goto defintion, takes you to the interface defintion & not ...