Track bike handlebars are 25.4mm diameter. Road bike handlebars are 26.0mm diameter. Sigh.

Lots of us want to stick a nice set of Nitto bars on a road stem, and this is the way to do it. It's just a thin bit of metal to pack out the handlebar to the correct diameter.

PS If you are feeling cheap you can, ahem, slice a piece out of a can of coke (or special brew... whatever floats your boat) and that will do the job nicely too!