why upcasting in java code example

Example: upcasting java

Upcasting is casting a subtype to a supertype, 
going up in the inheritance tree.
It is done implicitly
 in order to use method available on any
 interface/class, the object should be of
 same class or of class implementing the interface.  
   WebDriver driver = new ChromeDriver();
or
	TakeScreenShot ts = new ChromeDriver();