What does enable mean in Oracle DDL constraint?
not null is a constraint. The enable that follows it is optional if you want that constraint to be active (which is the default). So 

NOT NULL
NOT NULL ENABLE
NOT NULL ENABLE VALIDATE


all mean the same thing (validate is also the default, novalidate being the other option), so there really isn't an "advantage" in specifying it fully (except perhaps for documentary purposes).

If you wanted that not null constraint to be disabled, you could say:

NOT NULL DISABLE -- equivalent to NOT NULL DISABLE NOVALIDATE


or

NOT NULL DISABLE VALIDATE


See the Specifying Constraint State docs for the details and use cases on this topic.