The difference between a regular drill and an impact driver cannot be understated. You can use a impact driver for drilling holes, but it is for driving (and removing) screws & nuts where they really shine.
The jarring action that the impact driver inflicts on a screw allows the user to drive with ease. Regular drills, on the other hand, only provide a constant force. Drills are fine for small screws or small jobs, but if you are doing any serious driving you will never regret getting a cordless impact driver.