Even though Indy did flop there, screening your movie at Cannes does indicate a certain faith in your product however misplaced it might be. Unlike the exhibitors at Cinemacon, critics and audiences at Cannes don't have a vested interest in your movie's success so they're more free to trash it if they feel inclined, but they can also give a more legitimate-looking appraisal and help make promoting the movie easier. They clearly don't intend to just "dump" it.