I didn't quite understand at first - but this is something every smart phone should have. Battery performance of the BlackBerry Q10 is appalling out of the box (apparently, though, it gets better) - of course, the great thing about a blackberry vs an iPhone and some others is that you can have spare batteries and just swap them out when one dies. To do this sensibly, you need a separate battery charging cradle
This device not only charges your spare battery (and has a 'pass-through' to charge your phone from the same micro USB), IT ALSO DOUBLES AS AN EXTRA BATTERY PACK, and you can use the 'out' plug from the device to plug into your phone (or anything else with micro USB), and charge directly from the spare battery.
It's one of those 'why the hell didn't they do this before?' moments (perhaps someone has) - this combines one of those rechargeable charging devices you can buy with a swappable spare battery, and also means you don't need to carry an additional charger to power it up.
For what it is, I can't imagine it could be better.