what is an impact driver used for

An impact driver is a tool that uses rotational force to drive a screw and saves its bursts of quick power when it feels resistance. It is similar to a drill in that it spins a bit, but it uses an internal mechanism called a hammer and anvil to turn whatever you load in the collet. Impact drivers are especially good for uses like driving in large quantities of fasteners, longer screws, and lag bolts. They are also useful for driving screws into dense materials like hardwoods and metals. An impact driver is the fastening king of most tool collections, and it is especially good for driving 3-inch screws into wood, a task that is difficult for a drill even with pre-drilling. While you can use an impact driver in a pinch to drill a hole, thats not what it's best used for.