If they had the ship up and running and if they had won the Battle of Britain then the carrier would be useful during the invasion of Britain.
Anouther idea would be using the carrier to make a Dolittle type raid on the US. Bomb the US and land in Mexico and make their way back to Germany. If you can get B-25's off a carrier then you could get He-111 or Ju-88's off a carrier.
