While the appearance and the function of the two tools are similar, there are significant differences in what they can do. The impact driver relies in small impacts that help to drive the fasteners in while the bit is spinning. What this translates to is less fatigue for the user, because rather then relying on the torque like a drill, the impact driver uses thousands of small impacts to help send the fasteners home. With an impact driver you can easily drive a 3/8” x 2” lag bolt into sold wood without predrilling and with out twisting your arm off. Now with a drill, torque is how the power is used to drive fasteners, but what happens is when you are in the middle of sending a fasteners home, and something stops the fastener, the torque is felt by the user when the drill gets ripped out of your grip. It is a normal occurrence to strip the heads on fasteners with a drill since it is more difficult to control the torque when they slip. With an impact driver, the bit bites into the fastener head better and it becomes a rare occurrence when a faster head becomes damaged. A few other advantages to the impact driver is they are smaller in size and don’t use a drill style chuck but rather a standard universal quick change bit system.