One of the most common places I see it, is in Save methods of some frameworks.
The reason is that in many cases it is not actually possible to maintain the same object over a save call, if the object is being serialized to another machine and then comes back as a new instance (perhaps with some extra defaults). In that case you need the ref to make it obvious that the original reference is no longer valid.
As for its NECESSITY, I can't come up with an example where it would be required. Most places out is just fine.
