A crank radio uses an internal generator to create power. The external crank arm charges an internal battery by moving metal coils around a magnet, creating a current. This is especially useful for emergency radios because other charging options depend on sources outside of yourself.
Most radios have replaceable batteries and solar chargers. But a wall outlet is useless when the power goes out. Solar chargers only work when the sun is shining and there’s no cloud cover. And eventually, you’ll run out of batteries. Having a hand crank means that the life of a radio is indefinite as long as you have functional hands.