Better tell Charly to leave room for 3-4 inches in chest/back size
It's to attach the bag to a tiedown fitting or other suitable restraint device. One does not simply toss their helmet bag in the backseat of an aircraft.
This specific helmet bag is not designed for a detachable shoulder strap. The later helmet bags that replaced this design used detachable straps, but they were larger and it was warranted. It really isn't necessary for a bag this size and would only detract from the authenticity. This design could function as a s/s weekender depending how you pack, but it might be smaller than you think. A pair of shoes, some shirts, and an ipad/whatever would work fine. It is amazingly suited for a day bag.