Potentially multiple reasons for this type of capability - shorter take-off runs, additional fuel for longer ferrying, greater load-carrying capacity etc etc. Minor flaw is that the wing was supposed to be detached in flight with consequent risk of hitting one's own fin. That plus combat manoeuverability with the slip wing would probably not be the greatest - too much strain on the support struts - so it would make the slip wing Hurri a sitting duck in anything other than a benign threat environment (and there weren't too many of those in the world in the period 1939-1944!).