An impact driver is used when you have a seized fastener that will loosen only with brute force.
In wood, the hammer crushes fiber, freeing the thread by reducing friction on it while not disturbing the screw head. In steel, the hammer blow does little to disturb the material but fractures and crushes rust build-up as well as applying up to 200 ft. lb of torque with a single hammer blow.
This reversible heavy-duty driver is a good one. The large-diameter (1-1/2") handle has a textured grip, a hardened steel striking cap, and an integral flange to protect your hand against overstrikes.
Comes in a steel case with six bits: a 1-1/2" long #10 slot bit, 1-1/2" long #1, #2 and #3 Phillips bits, and 3" long #2 and #3 Phillips bits for extra reach.
The bit holder can be removed to expose a standard 1/2" socket driver (impact sockets recommended).
Made in Japan.