Light that comes into a scene off-axis from the camera view will ALWAYS look more dynamic, interesting and pleasing. It looks more three-dimensional, and it creates shadows on textures, shapes and form that enhance the visual appeal of the image.
And aside from that, using the flash off-camera prevents red eye and that horrible “deer in the headlights’ look that straight-on flash usually gives. You probably already know all this, though.
Of course, the main issue with using off-camera flashes is how to trigger them. Essentially, there are five different ways to trigger a remote lighting unit: