Basically I too have a requirement for Bi-directional Hash Map. One of the use case is we have is currently in our system one kind of naming say "A" namings, but some clients are interested in another naming say "B". Now for View Screens for another clients we need to map from schema(set) A to B . Alternatively if there is Data Entry Screen then in that case we need to map from B to A so the underlying system is unchanged.
Hopefully this should help. Thanks