my guess is for opening canning jars after they come out of the hot water. It seems my grandma had something similar.
^This. I think it's called a "ring vise" or "ring tool", for canning.
When I was growing up in rural North Carolina, we had a similar tool for "ring turning" - getting the lid rings off of Mason jars after they had cooled a little from the canning process. (The rings had to be removed so the lids could "pop" if they hadn't sealed. Otherwise, with the ring on, the lid might not have sealed, and the food in the jar would go bad without us knowing it. If the ring is removed as the jar cools, the lids can depressurize quickly, and the "pop" is a warning that one is not sealed.)
There was the larger "Lloyd-mouth" style, and the narrower "small-mouth" jars. The tool in the pic adjusts to handle both sizes, and others too.
Also, the tool in the pic looks a lot nicer than the tool we had. Ours looked like it was made to torture livestock or something.
~Chris