Napoleon's slow accumulation of power serves two purposes. First, he and the other pigs want to reap the benefits of "equal" entitlements without doing the work. They have that at the beginning, as they claim the role of "brainworkers" and tell the other animals what to do. Napoleon is not satisfied with this, and moves towards having military control of the other animals so no one can ask why the pigs do less work.
The other reason is for the pigs to raise themselves to the status of Human. Instead of working with the other animals equally, as Old Major wanted, the pigs alter the laws of Animalism and give themselves better benefits than the others.
They had thought the Fifth Commandment was "No animal shall drink alcohol," but there were two words that they had forgotten. Actually the Commandment read: "No animal shall drink alcohol to excess."
(Orwell, Animal Farm, msxnet.org)
At the end, Napoleon has become an honorary human, and has a farm full of slave animals to work for him. The other humans think nothing wrong with this, and are actually impressed that an animal was smart enough to play their own game; by gaining power slowly instead of trying to take it all at once, Napoleon trained the animals to accept his leadership instead of fighting it as they did to Jones.