Whether you're dismantling something and trying to preserve it, or trying to correct a project screw-up in mid-build, sometimes you need to remove a nail.
If you're pulling it out of trim, you can do that trick where you pry the trim away from the wall, then tap it back towards the wall to get the nails to reveal themselves, but this doesn't always work and/or you may damage the trim.
Other times the nail is buried too deep to get at, or even worse, you've accidentally snapped the head off.
Here master carpenter Matt Jackson, who's donating the door frame to his shop to Habitat for Humanity and wants to keep it in good shape for them, shows you how to use the tool. Note that he's pulling out burly 16d nails that have virtually no head: