That's funny: I had exactly the same question when I put a new master cylinder on my Stelvio last week (as the original one stopped working and it appeared that it is impossible to find a rebuilt-kit for this model).
Anyway, in the end I looked at the setup on my GRiSO and all I can think of is that it is meant to keep the 14-16-17 assy from rattling? I agree that a zip tie will most likely do the job just fine.