It slots into your steerer tube and extends past the cut point to give you more length to fit your stem onto. It's not the least expensive, because you left out everything else needed to install the OneUp version. On a bike, the rider is able to set their desired handlebar height by mounting the stem at the appropriate point on the steerer tube.

Commercially available steerer tube extenders can give you up to around 10 cm more length onto your steerer tube, which should more than cover any length lost from a previous cut. Steerer extenders should never go on carbon steerers because the stakes of the steerer failing without warning are too great; even in the case of a carbon steerer that was cut too short, they should be avoided because of the extra stress their pinch bolts put on the steerer. Can you put a new steerer tube on a fork?If you’re not confident in your preferred handlebar height and want to leave some wriggle room, add a spacer or two above the stem as well (you’ll remove these later). Lowering a bike’s handlebars gives you low and aggressive bike geometry, with stiff and responsive steering. So it’s a straightforward way to cycle faster. You’re likely to find yourself in this position if you’ve bought a second-hand bike and the previous owner has cut the steerer tube shorter than your desired length. Bike steerer tubes are most commonly made from aluminum, but are often carbon fiber for high-end bikes. From all comments, I conclud that what you carry, is to give you also peece of mind. Like protections. No one uses protection to fall!

If your steerer tube is already the right length, it was likely built to size by the manufacturer, (or perhaps already cut down by a previous owner if bought secondhand). Cutting your steerer tube can hurt the bike’s resale value, though, and this becomes more severe with a more severe cut.The steerer tube then extends from the fork, through the bike frame’s headset, to connect to the stem, which clamps around it. It is technically possible to remove a steerer tube from your fork and fit a new one, but it requires specialized tools, and you risk damaging the fork if you get it wrong. Optimizing your setup can be fun and engaging, and finding new ways to rework your gear for a better riding experience is rewarding. Add enough spacers to the steerer tube to set your stem to your preferred handlebar height. Then, add an additional 15 mm or so of spacers below.