The difference might not be clear but it is a bad practice to first down-vote. If an answer is incorrect/incomplete use comments and edits to help them improve their post. If they don't follow up and address the issues then down-vote. You might think that you can take back the down-vote after the post is improved but the effect is quite different. It give the new users that they are not welcome to the site and that is not what we want. It will also create a bad environment for sharing knowledge. A down-vote might not seem a big thing for you if you are used to the system, but it is quite a big thing for people who are not and might interpret a down-vote, especially without any comments to explain what is wrong as quite disheartening. We would like to make every user interested in the site feel welcome as much as possible. It also give a bad example for other users.
This is more important in the early stages of the site that we are building a community. The more experienced and knowledgeable users should help others improve and learn to write good posts.