Mental Ray includes two methods for bouncing light around a scene, Global Illumination (GI) and Final Gather. While the rays used for GI are narrower and give a more accurate result, Final Gather sends out big clumpy rays to light an entire blob of the rendering in one shot. Originally, Final Gather was intended to be used in addition to GI to lighten up dark corners and other areas that somehow didn't get their share of GI rays. But then artists discovered that you could get pretty good results with Final Gather alone, and that it was much faster to use than GI. So the use of Final Gather alone has become popular as a rendering workflow.