What is one of the strongest motivations for completing the rebuilding of the windmill?
In Chapter 5 when Snowball speaks to the other animals about the windmill, he explains that it can operate a dynamo which will supply the farm with electrical power to operate machinery which will do much of the work that the animals have been assigned on the farm.
Snowball conjured up pictures of fantastic machines which would do their work for them while they grazed at their ease in the fields or improved their minds with reading and conversation.
Certainly, this efficiency of labor appeals to the animals, although its construction involves a great deal of hard labor. However, once the windmill is constructed, Snowball tells them, "sordid labour" would be lifted from the animals; with this speech, the animals are won over and Napoleon utters a "high-pitched whimper" never heard before, and Snowball is run off the farm. Afterwards, Napoleon has his propaganda mouthpiece, Squealer, "explain" that Napoleon had never really opposed the windmill; in fact, the plans that were drawn upon the floor of the incubator were Napoleon's to begin with and had been stolen.
He had seemed to oppose the windmill simply as a manoeuvre to get rid of Snowball, who was a dangerous character and a bad influence.
The motivation behind Napoleon's continued building of the windmill is that with Snowball banished, he can now take the credit for the idea as well as its actual construction. For, Napoleon has known all along that Snowball's plan for a windmill is a brilliant idea as it will increase productivity on the farm; however, he has never wanted Snowball to be the leader and popular with the animals. Instead, he desires total control. Realizing that the idea of the windmill has been popular with the animals and that it will increase productivity and make life easier for the animals, it is well worth his while to re-institute its construction.
Snowball explains that, even though the windmill will take a lot of labor to complete, it would be better for them in the long run. The electricity created from the windmill would limit the amount of labor needed on the farm. The animals would be able to take a break from farm work.