The obvious answer is that you get the aircraft airborne quicker! If you wait until one rolls before the next one enters you're lucky to achieve the minimum departure separation on a consistent basis. However, if you can line up three at once at various intersections you can really shift some tin! Been there.. done it... eaten the stew... read the book... seen the film and even got the tee-shirt!! (My old mate ATCO2 will probably provide a more scientific reply).