The Lord now compares Jerusalem to a woody vine. In viticulture, grapes are usually only produced by vines that grew the previous year, so the old wood is pruned away to maximize yield. It’d be nice to be able to use that wood for something, but it’s kind of thin and twisty so you can’t really make anything out of it. Not even a peg to hang something on a wall. Instead it is just thrown into a fire for fuel.
The Lord intensifies the comparison by noting how the charred middle of the vine is even more useless after the ends have been burned away. One might normally expect the whole vine to be consumed by the fire, but it might make sense that the fire represents the earlier deportation of the upper classes, but the rest of the city’s inhabitants also have their judgement to go through.
You can make even the most useless things useful in Your plan.