That's the thing, they don't tension up like you think it would, but it will stay in place once it's set. I suppose you could sweat the line and slide it up, but there's other knots that tension a line better. What it is good for (talking about the poldo tackle here) is adjusting a sling length. I've only used it once or twice, but it works great for that. Here's where I've used it: we had a large and heavy pipe fab that we were trying to pick so we could fly it in place, kinda an L shape. The traditional way is to use 3 pick points, and use a chainfall or come along to adjust the middle leg, no big deal.
Only problem was that we had none of those tools, and even if we did it was likely over their load capacity. But we did have lots of straps, mainly the big tubular endless slings, aka belts, which are very flexible for their capacity. By using a poldo tackle (rigged with shackles) i was able to infinitely adjust the sling so it was the correct length, and was even able to slide the outer straps back and forth so it would pick level in both directions. Basically set it where i thought, lift, lower, adjust, then lift again til it was right. Once it was loaded i used some tag line rope to bind the works all together to lower the risk of it sliding, but it worked great and we were able to accomplish what we needed to. We used rated slings and shackles, and since it's completely locked in place it's an approved method for doing something like that.
Lots of objects that need rigged have the center of gravity not in the middle, and doing industrial work you will likely be using straps to lift them, since they have an actual load rating and don't rely on knots. Even using rope it's very hard to get the lengths exactly right, so the poldo tackle makes an adjustable strap in between the doubled over length and the tripled length. And the best part is that you don't need to undo anything to adjust it, you simply slide it and come up again, so it's much easier than other adjustment schemes. It's kinda sketchy because you rely on friction alone to hold it in place, but a quick lashing or clove hitch can secure it enough to use, because depending on the sling material unloading it could cause it to shift.