Here are a few examples of what I am talking about (I broke out the digital camera for these... I know, my webbing and ruck are filthy, shhh, don‘t tell my CSM!)
First, here is a properly assembled buttpack...
In the bottom picture, the straps run through the O-ring, which centres takes the weight off the plastic clips that hold the buttpack on to the webbing straps.
Here the O-ring is by-passed. I see this all the time, and actually once had to correct buddy during a brief halt in a ruck march, as he was all messed up and losing his kit, etc. His buttpack was coming off, the plastic clips can‘t support much weight if the strap doesn‘t go through the O-ring.
Now the ruck... it might be hard to explain here...
Ruck with no valise, you can see the straps run through the O-rings on the bottom of the ruck bag, then through the frame and out the holes in the bottom of the frame. When the valise is packed in there, the valise straps hold it tight against the frame, which keeps the weight from shifting around.
Here, the ruck with valise attached incorrectly has the straps NOT going through the frame, just the O-rings and the bottom of the frame. The straps touch the valise on all sides, and there‘s a gap between the frame and valise. This means it shifts around, no matter how tight you make the straps. After a few klicks on the march, this tires you out because the weight shifts around as the valise bobbles to and fro, and it‘s less secure, as the valise slips out of the straps sometimes.
Hope that makes some sense...