G15 Knurled Front Ring
It can be assigned to various functions, as described on the page 168 of the user's manual. Which one depends o the shooting mode used. In A mode - aperture change, in S mode - shutter speed, I use it in P mode to set focal distance in steps.
I think you may be referring here NOT to the Black Ring but to the FRONT knurled edge rotating dial just forward of the shutter press button as that can be assigned values as you say.
I believe the OP refers to the main Black Ring which encircles the lens at the base.. THAT is simply decorative in the first instance but is there so it can be unscrewed (pressing the press-button about 5 o'clock position to the lens when looked at in frontal view..that Black Ring unscrews and the exposed thread is then used to screw on such as an adaptor Hood that encircles the lens itself and enables you to screw into the front end any sort of Filters you may wish to use.. you cannot do this of course on the LENS itself.