Off screen sounds are generally produced in the post production stage, and not during the set while shooting the particular sequence. The characters surely will portray that reaction which is needed from the director, and it's mentioned in the screenplay as well when the sound will be heard, but in reality that sound is either imagined by the character, or someone else might produce the sound for the character to react to the scenario, and then the original sound is added later on in the post production stage. Off screen sound is added for various kinds of reasons depending on what the director of the film wants to create on screen. It can be used to add details about the particular location, for example, a forest where the sounds of different animals can be heard. It also can be added to show the space of the location, for example if some object falls, the echo, can be created as well. This sound also generally adds a realistic feeling to the situation which every director wants. Even in horror films mostly off-screen sounds are there for building up the the fear, and as well as the curiosity for the audience. This also helps adding a meaning to your on going action. It plays a role in creating a world outside the visuals which is seen on screen. It makes your audience hear what your character hears. The off screen sound tells a story with the sound produced itself, like in a blank screen the sound of a blast is heard, the audience itself will understand that there is blast being taken place. It also let's you get into the mind of your character, and know what is going on into their mind.
This also gives us a creative liberty, and makes your audience enjoy what's happening on screen, even though it's heard off screen. The progression of your story also takes place due to off screen sound also like by introducing a new character, or even indicating something.
Off screen sounds are creative, exciting, as well as making your story enhance with on going action.
