This small grommet allows you to use a presta valve in a Schrader valve hole without cutting or herniating the stem. I use Schrader-ready rims on all our touring bicycles because in the middle of nowhere, you never know what kind of tube you will find. At the bike shop, it will probably be presta. If there's no bike shop and you go to a Walmart or similar, it will be Schrader because Schrader inflation methods are a lot more popular. Keep one of these grommets in your patch kit (they weigh like 1.5g) and you are ready to use a presta tube if you can't find a Schrader. Or drill out your rim hole to 8mm (and file burrs carefully) and run it from the beginning with your presta tube and you're ready for a Schrader. You get peace of mind knowing you can use whichever tube you can find.