This is an adjustable shim designed to adapt mounts designed for 31.8mm bars to 22.2mm bars.
Specifically this part was designed to facilitate the use of a Shimano GRX BL812 inline brake lever on a smaller 22.2mm bar. This lever also rests on the bar, which is why the shim is so long. You may wish to shorten it depending on your use. The longer shim section also facilitates using a hose clamp to make sure it's nice and tight where the resting part is. The adjustable nature of this part allows the whole lever to be moved closer or further away from the bar than would be possible with standard circular shims.
I'd recommend printing with 100% infill for durability.