These Buddhas in manifestation from all over the universe wanted to clarify that they are one with Śākyamuni Buddha. Śākyamuni Buddha is like the moon up in the sky; those Buddhas in manifestation are like moons reflected on the water. The land of Śākyamuni Buddha is this Sahā World. If the Buddha, the moon in the sky, does not move, the Buddhas in manifestation do not move either. They live in this Sahā World to protect the practicers of the Lotus Sūtra just as the retainers living in the Sahā World pay respect to their lord, and parents love an only child.
Shimoyama Goshōsoku, The Shimoyama Letter, Writings of Nichiren Shōnin