Result sets are deleted automatically after the z-association is closed.
Servers often have a limit to the number of named result sets they can store.
A server may unilaterally delete a result set, particularly when the limit is reached. So it makes good sense for a client to delete a result set it no longer needs, to forestall the possibility that a result set that it does still need may be unilaterally deleted. Some servers charge to maintain results, so deleting a result set as soon as it is no longer needed may help reduce cost.