They are quite useful if you make your own pickles or oil-packed vegetable (like artichokes).
You see, you want to stuff as much as possible into a jar, but if you overdo it some part may manage to surface and come in contact with air. And then it goes bad.
The solution is the little pizza box lid lifter (catchy product name, BTW), but placed upside down - with the flat part pushing on the pickles and the three little legs pushing on the jar lid.