Some battery-powered devices are so poorly designed that they consume power even when switched off. However, if these are devices that you rarely use, the batteries are usually already empty when you want to use the device.
Of course you can remove the batteries immediately after each use and store them outside the device. But first of all you have to think about removing the batteries after every use and secondly external storage is highly impractical.
With this adapter, the device is always ready for use - as long as power is available from the wall socket.
BTW: I used bare silver wire (0.6mm) for the battery contacts.