Why does Napoleon insist the windmill must be rebuilt immediately?

Your question seems to relate to the incident, at the end of chapter six, when the windmill has been blown apart by devastating gale-force winds during November. The animals are obviously shocked that the "fruit of their struggles" has been demolished completely. Napoleon sees this an ideal opportunity to further demonize Snowball who has been banished from the farm. He immediately blames Snowball for this terrible ignominy. Evidence, in the form of the footprints of a pig leading towards a hole in the hedge, supposedly proves Snowball's culpability. Napoleon calls him a traitor and insists that the animals should immediately start rebuilding the windmill.
Napoleon's demand is to ensure that the animals regain hope in their ability to complete an arduous task. Furthermore, since he has claimed to have developed the idea of the windmill himself, the animals' commitment to completing such a difficult task will be evidence of their loyalty to him and, in general terms, their desire to be utterly free of humans and become completely self-sufficient. The completed windmill will provide electricity and make the animals' lives much more comfortable.
In addition, Napoleon wishes to belabor the animals with hard work so that there is no opportunity for them to question his and the pigs' rule and their discriminatory tactics. The general animal populace will be too exhausted to ask uncomfortable questions. They will also seem less disciplined and disloyal if they should not throw their weight behind the completion of such a crucial project. Any laxity on their part would allow criticism from other comrades and they would, like Snowball, be deemed traitors to the principles of Animalism.
Napoleon's demand is, therefore, a sly stratagem to, with the other pigs, maintain their dominion over the other animals. The general animal populace has no choice in the matter, and the hard work they do ensures that they remain servile while the pigs remain in control.