As an event planner, my job is to make sure all vendors understand the host's vision for the event. You might not think it is important that your DJ knows you are having a "rustic garden theme" wedding, but the more information that is provided to your vendors, the more smoothly the event will run.
If you are hosting a black tie gala, make sure all of your vendors know it is black tie. Sometimes photographers and bands dress more comfortably because they will be moving around a lot. Communicating the dress code to them is important. A good event planner will usually take care of this for you.