Monitors are used in vehicles where a display is required and the common example is a Reversing Camera display.
In many new vehicles the display is part of the in-dash unit and often serves multiple functions which can preclude the use of this for anything other than short term viewing, such as when reversing and as such a separate monitor is seen as the best option when adding adding additional cameras such as with a caravan rear vision system.
This allows the in-dash unit to be easily used for entertainment or navigation while still having vision from an additional camera.
When selecting a monitor always ensure that the mounting option is suitable for the vehicle and use intended and check that the camera/cable being used has suitable connections. All monitors are supplied with wiring harness for power and AV connections (either 4 Pin or RCA) and adaptors to convert from 4 Pin to RCA and vice versa are available, see accessories.