Actually that pin has also been used in some of the Performance Center K and L frames, so it's not exclusive to the N frame. However, it is standard to all of the N frames that I am aware of and probably the X frames as well.
As noted it's primary purpose is to provide a trigger stop. However, IMO it also helps reduce buckling of the rebound spring inside the bore of the rebound slide. As a result it's been my experience that revolvers with this pin have smoother DA triggers with less stacking.
Now a warning. These pins are position critical. If you look closely you'll note that one end is sort of pointed. If you find that your trigger locks up solid with the revolver held in certain positions you'll need to flip that pin end for end. BTW, I have a vague memory that the pointy end goes against the pin for the rebound slide but don't count on that memory. To tell the truth I got it wrong so often installing that pin that I've made all of mine "double ended" so they cannot be installed wrong.
