I hate it when I plug something into something and I've no idea what is going on. Is the device taking any power? How much? Is the supply holding up?
Some time back I ordered one of these. The price was amazing (and delivery was free). It arrived today and it works a treat. The case is a bit flimsy, but I'm not going to submit it to anything too rough, and so it should do just what I want.
What does it do? You plug it between a USB device and a power source and it tells you the voltage that the source is providing and the current the device is consuming. It can tell you if your phone is charging or not. It can tell you if your Raspberry Pi is taking too much current out of the adapter. For the price, well worth picking one up and adding it to your toolbox.