When whales flip their tails in the air its called "lobtailing." Some scientists think it's a non-vocal form of communication, but the sound isn't heard very far compared to vocal noises whales do sometimes. So, some scientists think that whales do lobtailing as a visual way to communicate, but this form of communication can also be heard a little. Scientists think it may be an aggressive behavior, but the whales may also use it to catch fish by frightening them with a loud noise. For more on this kind of whale behavior, see this Wikipedia article:

Maybe for mother/calf communications (calves flipper slap to play or attract mother's attention), social communication (dominance, aggression, or 'hey, I am over hear'), may be to relieve an itch, or just for fun.