Firstly, Google Instant is a hidden feature and they provide no cues for it before you begin typing. They can remove the button after the user has begun to type because at this point instant search is exposed to the user. Having no search button available before exposing a hidden feature would be potentially confusing to the user.
Secondly, Google Instant is an optional feature and can be switched off in your search settings. If you have the feature switched off you would need the search button.
Thirdly, as others have already pointed out Google Instant isn't always available. The button needs to be available in case Google Instant is on but doesn't work.