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If the camera has interchangeable backs you can change a film mid-roll so you could shoot colour and black & white within seconds of each other. This is also useful if you have several users using a camera - each could have there own film back. It's also good to record specific subjects on the same roll of film. For example you're out walking and you could take flowers on one and landscapes on the other. Many medium-format cameras have the option of changing format with different backs. You could, for example shoot 6x7cm, 6x4.5cm and 35mm from the same camera using three different backs. Polaroid backs are also available for many cameras. This means you could shoot a Polaroid print and instantly check lighting, composition or exposure and then replace with the conventional film.
A medium-format camera uses roll film to take pictures with 6cm width. These often have interchangeable backs so film can be swapped mid-roll and Polaroid or a digital back can be used to instantly proof a composition or exposure. Medium-format cameras are costly and often bulky so tend to be used by enthusiasts or professionals.