- When you don't have a drill around.
- When you do have a drill around but you have it ocuppied in some thing else. 2a. When the you are using the drill for drilling, you can use the impact driver for driving screws; that way, you don't have to keep switching between drill bit and screw bit on your drill which can be tedious.
- Don't go crazy as to which one to use - just grab the one closest to you and after a little while, you will have your own personal favorite system all worked out.