We're working out what we think - we'll add our view here!
Granite Gear says:
Here is the pocket you have been asking and waiting for! The Add-On Pocket fits both your shoulder straps or your belt and will hold your cell phone or even the plus-sized smartphones within easy reach. Don't want your phone this handy then use the pocket for sunglasses, snacks, first aid kit or anything else you need readily accessible. This is a great addition to your pack.