If you look closely, you’ll see a little indented line along the washer that the cable is pinched against as the bolt tightens. It is imperative that the cable runs along this line. If you think about basic levers and pulleys and such, the fulcrum or contact point is a major determinant to how the machine works. If the cable isn’t routed properly, the derailleur simply won’t function well (if at all). FWIW, this detail is more crucial for the front derailleur than the rear – I’ve seen a lot of horrible shifting solved simply by a 5 second fix.