Umm, multiple ground wires maybe? It's not required but it does make the job a lot easier. What would you rather do, drill & tap a bunch of holes for ground lugs or screws or just one to mount a ground bar that could take up to a dozen ground wires?
Not required. Who says it is and what is their reference in requiring so? If a local rule then I guess you have no choice, NEC doesn't require it, it is just convenient in many cases.
I assumed we were. Where does it say you must have a "ground bar"?Maybe we're talking about in a sub-panel?