Hello, roots.
I think you add the final modifier when you completed a class and you know that there will be no need for it to be the super class of some other class. For example: Let's say Planet is a subclass of Body and Earth is a subclass of Planet, but you do not get different types if Earths as you get different types of planets. Thus, the Earth class must be declared final. I think the purpose of this is to improve performance.
Hope this helps.
