You use:

  • public scope to make that property/method available from anywhere, other classes and instances of the object.

  • private scope when you want your property/method to be visible in its own class only.

  • protected scope when you want to make your property/method visible in all classes that extend current class including the parent class.

If you don't use any visibility modifier, the property / method will be public.

More: (For comprehensive information)