Sometimes it's not possible or really tedious to get a USB cable to the USB connector for power input on a Raspberry Pi. Since the 5V pins, the USB power connector and the USB hub share the same power rail, it doesn't matter where the electrons enter and exit. The only difference is that the Pi has a few capacitors, a resettable fuse and a diode directly behind the USB input. When powering the Pi via the 5V pins on the 40pin header, this protection and the capacitors to deal with sudden power draws won't work.

To make back-powering the Pi via the USB hub a bit more convenient, I made these backpower adapters that contain the same resettable fuse, capacitors and diode as the Raspberry Pi design.