This 2mm-thick spacer allows you to attach the Peak Design Capture v3 camera clip to thin straps without the bolt protruding through the backside and into your clothing/body. It features a hole that prevents it from separating from the clip when attaching/detaching Capture clip to your strap. It also has keying to keep it perfectly aligned at all times.
Fully seated, the standard bolt would protrude only ~0.4mm, so even the thinnest straps would work fine. Using it with other bolts (e.g. the Peak Design-supplied long bolts) will also reduce the protrusion by 2mm.