When I built the Le Requin Xebex I had a dozen of these. They are for joining running rigging to rigging belays. For example, a short rigging belay, usually a heavier line than the running rigging line, would be anchored to a bulkhead with a loop at the opposite end. The loop would be served around the center of the toggle. Then a rigging line would be looped around the toggle and run for whatever purpose. The toggle served as a joint in rigging lines that could easily be undone without breaking down the more permanent anchor belay to the bulkhead. Make sense??