The first time a new C30 owner collects their clothing from a dry cleaner, instead of a hook above the back seat, they find an accessory slot -- not useful. These hooks fit into that accessory slot to provide a place to hang dry cleaning. The opening of the hook is also wide enough to hang a sport coat or suit jacket.
There is a long and a short version of the hook. The bottom of the longer version can be seen from outside the vehicle through the back window so that you can hang your cleaning without craning your neck inside the door to see what your doing. However, the long hook is long enough that you may want to remove it temporarily if you are going to have rear-seat passengers.
The short hook won't be an issue with passengers, but requires a bit more effort when hanging clothing because it cannot be seen easily through the rear window.
The long hook is 4 cm (1.57 inches) longer than the pictured short hook.