Official charger rubber thingy

[Stefan]

Just read in another thread something with the "official charger" so this came to my mind:
Does anybody know what the little rubber thing around the USB cable should be for?
At first I thought maybe someone did a mistake while designing the charger, so that there might be a dangerous voltage accessible somehow. But after close looking and poking some measurement probes at the USB port I couldn't find anything unusal. (would've been a really hacky fix anyway)