The eventual meaning behind the windmill is another perfect proof of Napoleon as dictator.  Napoleon is always pulling the strings behind the scenes and orchestrating all things to his own benefit and NOT to the benefit of the other animals (as he makes them believe).  The windmill is no exception.

Of course, the windmill is not Napoleon's original idea, it was Snowball's.  However, it isn't long before Napoleon sees the benefit of the windmill for his own use.  Snowball proposes the windmill to ease the animals' heavy burden of work.  The original hope was for the windmill to produce "electrical power."  Snowball, of course, hopes that, as a result, the animals will have more time for fun and enjoyment.  The unfortunate truth is revealed here however, for the windmill:

had not after all been used for generating electrical power. It was used for milling corn, and brought in a handsome money profit.

So much for the bereft ideas of Old Major:  that shared work would benefit the masses, ... instead, this shared work simply makes more money for Napoleon.  The animals are slaves to Napoleon and his cronies.  Not only is the windmill being used to grind more and more corn, but Napoleon demands a second windmill to be built, again for the dictator's own  use.