Why does every object in Java implicitly extend java.lang.Object class?

I would say that the reason is to have a common API for all objects in java to supports basic functionality like

• synchronization – wait, notify, notifyAll
• garbage collection – finalize
• collection support – hashCode, equals
• object cloning – clone

And every object

• has a class it belongs to – getClass
• can represent itself as a string, because we are
  humans and can read strings – toString