It's used to help align the wheel bolts when you change the wheel.
You put the grey plastic end in one of the wheel bolt holes in the hub, and then slide the wheel over the shaft. Once the wheel is sitting on the hub, you can put the bolts in without having to jiggle it around to get them lined up. When three are in, you just pull the shaft out and put the last one in.
Those Germans think of everything
Ian.
If it aint broke - Modify it...