Oh, I see. I would guess for religious purposes, i.e. idols. Or, maybe they were there to ward off any who might think of visiting the island. Could you imagine seeing those giant statues and thinking, "Maybe we should go to a different island."
We don't know for sure, but they had a significant religious history to them. Rapa Nui is fascinating for their unique culture, independent writing being developed, and cultivation techniques used to settle the entire island.