Self-seeding may explain why large tumors tend to grow (in percentage terms) more slowly than small tumors: It could be that growth is a function of surface area rather than volume.
Among other mysteries, self-seeding may explain why tumors sometimes regrow in the same location after being surgically removed: not necessarily because surgeons failed to remove part of the original tumor but because some itinerant cancer cells returned later to their original home to start a new tumor in the same place.