The Raptor's design cuts the visible area of the immediate exhaust by ~30-40% from directly aft, and by a huge amount when viewed from a small off-angle. That could be a significant difference in rear-hemisphere detection. I could imagine IR acquisition denial or delay as well, which could mean the difference between death or a fighting chance when talking about a high HCA/AOT IR missile shot.
Great selection of pictures, though, Weta43.