All of Indonesia is predominately Muslim with the exception of Bali being mostly Hindu. I have learned that each year the island builds these monsters in celebration of the Hindu new years which is on March 5th, the same day I fly home to Colorado. From what I have gathered, the folks here believe that after midnight on new year the "spirits" fly through the air looking for signs of life. What they do when they find it I have no idea, nor do I really know what part these sculptures have in detouring the spirits. Basically, I know very little, close to nothing, about this event except the people here eat, play music, and generally celebrate up until midnight then lock themselves at home and don't leave for 24 hours. The whole island is hiding from the spirits for the first day of the new year. Nobody is on the streets. Nothing is open. Lights are completely extinguished, although I heard guest houses will have a few candles burning for the westerners, but they are not allowed to leave their hotels. It's a complete lockdown for a full day until the spirits have pass over the island.
Again, I may have this all wrong, but I'm not here to spend my time looking up facts on the Internet. I just think it pretty cool that there are half built monsters everywhere. I hope they are all set on fire and burned at the end.