...
This idiom can also be suitably used by classes designed for inheritance. If a superclass thread requests a lock on the object's monitor, a subclass thread can interfere with its operation. For example, a subclass may use the superclass object's intrinsic lock for unrelated operations, causing significant increase in lock contention. Also, excessive use of the same lock frequently results in deadlocks. This idiom separates the locking strategy of the superclass from that of the subclass. It also permits fine grained locking as opposed to coarse grained because multiple lock objects can then be used for seemingly unrelated operations. This increases overall responsiveness of the application.
...