Hacking wall wart as a USB charging station
Many devices won't properly charge from a 5V supply, they need a USB host (or power controller) to supply the correct command first.

The original USB spec (USB2.0) gave only 100mA per port until the device identifies itself and provides it's power needs to the host.

Then the current is limited to 500mA.  USB3.0 increased this to 1.5A, again through negotiation.  Some devices adhere to this spec so won't charge (at more than 100mA) until a host tells them so.  Later specs (as well as deviations from the spec) again increase the available current through negotiation for battery charging by using the data lines as power lines to increase the current capacity of the  wire.

Basicly this means that sometimes pluging some devices into a wall wart (PSU) will only change very slowly (if at all) unless that PSU has a USB host or power controller inside it.

I have an Asus tablet that certainly behaves like this.