The rounded profile headtube of the Pinarello Dogma 65.1 frameset was designed for pro-level cyclists. As such, the headtube is fairly short for recreational cyclists. Most pro riders are limber and praticed enough to ride with a minimal stack height, even on shorter head tubes. While the effective safe distance of the steerer tube is approximately 80mm above the top headset bearing, and having a taller headset top cover doesn't change that, having the options of various stack heights from the cover allows you to have a cleaner, more pro-looking setup. Please bear in mind that this is effectively a headset bearing cover, not the "cap" that sits on top of the stem for bearing adjustment. Pinarello names this part the Dogma 65.1 Carbon Headset Top Cap, so that is what we call it, but it in US cycling circles, that is a bit of a misnomer

These carbon top cover are machined and shaped to integrate cleanly with the Dogma 65.1 frameset. The pros generally opt for the flat 3mm version and slam the stem on top and call it good. Most of us need a little more height, so Pinarello offers two tapered sizes, 17mm and 25mm so the general public can have a clean look as well. The larger sizes feature two gaskets that serve to seal out moisture and ensure even fit on the steerer tube. The 3mm version has only a single gasket that prevents infiltration to the bearing. The gloss finish and Pinarello logo's match the Dogma 65.1 scheme and make your life simpler when dealing with stack height. And who doesn't love that?