As one reads this allegory of George Orwell, it is important to keep in mind that the author is really telling the story of Communism. With this ideology, there were those like Leon Trotsky, represented by Snowball, who were brilliant and sincere about improving conditions for workers, while others such as Josef Stalin, represented by Napoleon, were more concerned about political advancement and changes in government and their personal power.
Under the direction of Snowball, then, the animals begin construction of a windmill which will provide energy: heat in the winter and electricity to run some machines which will ease the burden on the animals.
Snowball conjured up pictures of fantastic machines which would do their work for them while they grazed at their ease in he fields or improved their minds with reading and conversation.
Thus, the windmill is symbolic of modernization and progress from the "peasantry" of the animals whose manual labor is all that runs the farm. Communist leaders strongly felt that electricity was essential for their new nation as the infrastructure of the Czars was yet feudal and Russia was not keeping up with the West. Snowball, like Trotsky, yearned for Russian progress.