it's because those that have to do something in response to the order repeat it so that the Captain knows they heard it correctly.  A submarine requires coordination among men (and now women) manning various stations, and if one person misunderstand the order, it could fuck up the whole operation.  If four people are supposed to do something simultaneously to carry out the order, the Captain wants a confirmation that they received and acknowledged the order.  

 
They used to have submarines at Disneyland, so I have do some experience.  Lol

Remember this is a war ship and not some poker game. The stakes are massive. In the heat of the moment and sometimes with more noise than a Hollywood sound stage would account for in real life, commands can be misunderstood or not heard properly, either of which could lead to a miscommunication that could doom the mission/vessel.  Repeated commands just lessen this possibility.