A manual impact driver requires you to hit something hard with a hammer, if that something is in a weak structure you might find that you've just removed said screw/bolt/whatever by driving it straight through the body of whatever the bolt/screw/nut/whatever is in. Electric is basically the same mechanism as a hammer drill, a couple of spring preloaded ball bearings stop the bit from spinning and that impact is the impact you're using to undo the screwboltthing.