why does napoleon let snowball complete the plans for the windmill before driving him away?

Napoleon allows him to complete the plans because the windmill is a great idea, and he knows it. Snowball was the most capable of drawing up the plans, but Napoleon doesn't want to have anyone else being looked up to, thus, like any true dictator he gets rid of his competition and takes all of the credit himself. For Snowball to have taken any credit would have diminished Napoleon in the eyes of the other animals; he wouldn't let that happen.