They’re used to overcome poor flue draft. Having a look at the photos a couple points. Any bend in a flue causes a restriction, over here if we need to turn 90 degrees we try to utilise two 45’s, although a lot of older systems still have 90’s. Also why does your flue terminate by coming back on itself? Is this some ludicrous attempt at stopping downdraft? Surely this could be aiding in any poor draft you might be having? I’m sorry but I’m struggling to make sense of all your queries