Normally "cryptogenic" means the same as "idiopathic" - that is, without an identifiable cause at the moment. It is useful when one uses it to state that no cause was found for the phenomenon, but one must take care not to use it as "without a cause/reason" because of course there is always a cause for medical phenomena (not identifying it means only that it escapes our current capabilities).

It is useful because amplíes that is unknown the etiology by the actual moment

Yes it is useful, cryptogenic means we had evaluated the disease thoroughly but unable to find a cause. So it's an entity we compare with known etiology to find a prognosis.