In manga, a background does not only serve to provide atmosphere and make the more important elements look pretty but also tells the reader where an event is happening, what time it is and other small messages. Even emotional changes in a character need the background to match it so that the reader could feel it. Every manga sketches should have their own background to emphasize the environment of the action is.
Types of Backgrounds
With regards to settings, we can basically separate manga by modern realism, historic or fantasy. Backgrounds however, can be separated into cities and buildings, natural environment and abstract (meaning fantasy backgrounds).
Of course the last but not least, is the atmospheric type. It doesn't really express any location or passage of time, but uses patterns and differing depths of letratone use to create atmosphere.