Not sure what the real question is. You could be asking one of two things.
1) The hammer function on a hammer drill is of no use for putting in screws. The hammer action is along the bit (into the material being drilled), and might even make it more difficult to screw with the drill as it will add additional friction (you probably would not notice it). The primary function of the hammer action is to 'pulverize' morter/concrete/brick to allow the flutes of the drill bit to remove the material when drilling holes into these materials.
2) On an impact driver, the impact action is rotational. When the resistance is low, the impact driver works like a drill and does not provide the impact action. Once resistance above the basic motor capability is met, the impacts start (like a little hammer, 3,000 times a minute). This is in the direction of rotation and so helps to seat a screw. The result of this is: a) The tool does not twist in your hand when resistance is met, b) Provided you keep some force pushing on the screw, there is much less chance of the drill bit camming out of the screw head, and c) You get much higher effective torque to seat a bolt/screw, for example, whereas 18 volt drills advertize torques in the range of 400 in-lbs, 18 volt impact drivers advertize torques in the range of 1400 in-lbs.
I love my impact drivers, but find there are a couple of disadvantages: a) It has too much muscle for small cabinet screws, b) I've not had this problem, but you can twist the head off screws if not careful, and c) they are noisy.