Napoleon uses the windmill to his advantage by distracting the animals with it to keep them busy while he garners power for himself.
The windmill was originally Snowball’s idea, and Napoleon was against it from the beginning. The windmill divided the animals into two camps. After running Snowball off, Napoleon claimed that the windmill was originally his idea, and Snowball stole it.
The windmill is the promise of an easy life for the animals. Snowball convinced them that they would have machines to do all of their work, electricity in their stalls, and a four day work week. Napoleon continued to encourage this dream, because it gave the animals something to focus on other than what he was doing.
He did not give any reason for having changed his mind, but merely warned the animals that this extra task would mean very hard work, it might even be necessary to reduce their rations. (ch 5)
The animals spend long hours working on the windmill. Every time something happens to it, they rebuild it. It keeps them going, giving them motivation to work. Napoleon uses this to keep the animals in line, as a combination threat and reward. Ultimately, the windmill is blown up by the humans and they decide to build it again, making the windmill a perpetual un-granted promise.