The purpose of Exception mechanism is to notify user(end-user) of class (interface of method) that there are special situations or conditions he or she must at least know about when using such piece of code.
If you declare Exception and then don't throw it for now (but still planning to throw it in the future), the users of your code can already use it for now, and don't make any changes in the future, when you will finally change your IMPLEMENTATION (not interface = contract) so it will throw the Exception.
