Napoleon needs the plans because he needs the windmill, but he has neither the intellect, the ability, nor the skill to make one all by himself. He is willing to use Snowball to reach his goals and then throw him under the bus, so to speak, and take what he wants from him in the process (presumably, he could also take the credit as well). Napoleon is a very self-serving character. He is only out for his own needs, wants and desires with very little consideration for the animals under him or for how much work they do or what they give up so that he can have what he wants to have. If he had driven Snowball away before the plans were completed, then he would not have had the plans. Even if he decided not to use them, he would still have them, and for a character type like Napoleon's ultimately the having is what really matters.
In my opinion, it is because Napoleon really wants the windmill and because he does not really have much of an idea of how to build one himself.
Napoleon is really smart politically, but not that great in a lot of other ways. So he knows that he can't build the windmill himself. But he thinks the windmill will be a good idea. So he has to let Snowball get the plans all made and then drive him off.
That way, Napoleon will have the plans and Snowball won't get any credit because he won't have started the windmill and Napoleon can claim Snowball was never really in favor of it.