If the forks turn hard to either side, the top yoke can be damaged as the steering stop is cast as part of the top yoke. The telelever and fork legs can also be damaged as they come together once the steering stop breaks. This is not usually a problem, but can happen if the bike falls hard in an accident or off-roading. The larger piece wraps around the front of the telelever and softens the impact whilst acting as another steering stop, so protecting the top yoke as well.
Any clearer? If not, look at the Touratech catalogue where there are pictures which might help.