Event files usually have more data than can be expressed in a simple 2D image. Usually one filters on time and/or energy and they are integrated over to make 2D images. ds9 allows for 3D datacubes to be displayed as a series of 2D images. This make it easy to make spectra and lightcurves for many points in the image -- even if at a low resolution. Note: ds9 can actually support more than 3 dimensions, and ds9 version now has support for 3D rendering with full pan & tilt functionality.