Changing the initial launch position makes it easier to choose what area of the planet the periapsis will be over. When I launch the birds, unless I take great care to do otherwise, my periapsis tends to slightly after my launch position.
With periapsis's in different positions, you can get birds into rarely colliding 'resonances' of sorts. Making sure that the periapsis of birds is not over the default starting location is also good to ensure that you do not lose orbiting birds to a pre-launch bird.
