Sometimes IDENTITY columns need to be cleaned up a bit -- their values updated or re-ordered, for instance. This can occur when data replication strategies fail, or when unexpected data was inserted. This is a somewhat rare occurrence, but when it does happen it can be extremely painful due to the fact that IDENTITY columns cannot be updated, and the property cannot be enabled or disabled. As a result, in most cases doing what should be a simple update requires creation of a new table.
Likewise, we sometimes want to be able to turn an existing column into an IDENTITY column. The same problem exists there -- it simply can't be done.