The aspect ratio of any projection surface matters if you like to make most of the projector's resolution and brightness. The chip inside the projector has a specific resolution and aspect ratio. That chip produces the image when light is either send though it (LCD projector) or reflected off it (DLP projector) and subsequently directed through an optical lens system to the projection surface. Chip manufacturers give us designers only a couple of different aspect ratios to choose from. Basically they range from a classic TV ratio (4:3) to the modern high definition TV ratio (16:9), so from almost square to rectangular. Within those ratios there is a confusing number of possible resolutions. You can see resolutions that correspond to the common projectors chip ratios in the picture above. The goal for the designer is to fit as much resolution as possible onto the projection surface. The aspect ratio of the projections surface may however not fit any of the availble ratios or resolutions. Crucially regardless of the resolution of the artwork, the resolution on the screen dictates what resolution the audience sees!

The solution to that is to either over-project and waste some of the projector's resolution and brightness or to devide the surface up between multiple projectors. The illustration below shows an example of the type of choices a projection designer has to make sometimes for a single projector.