It has to be set from java as I don't think it's in the resultset library.
It allows the resultset to be updated and reflect changes back to the underlying database.
Whereas now we have readString(...) you gain updateString(...,value).
If used with SENSITIVE , the resultset is 'live' and will change with outside database changes.
Not too hard to use, but you have to weigh up the extra effort involved against using select then update queries.