Impact drivers look like power drills. But on drills, the only thing that moves is the bit. On an impact drill, a fast-moving hammer continually pounds the bit, letting you ease up on the pressure you would normally have to exert on the drill.
You can easily screw down boards, planks and other pieces of wood with less effort.
Different types of bolts and fasteners can be applied with an impact drill.
Impact drivers are a great help in tight spaces, such as when you're hanging windows and you might not be able to get the leverage you need to exert a great amount of pressure.
The impact driver is not an end-all. It can be used most successfully for general fastening and up to medium-level construction jobs.