Perhaps the best way to think about your low back is that it is like a spring. Any good spring, when you compress it, neatly absorbs the energy you use and, when you let go, springs back to its resting length.
Your low back should do this too. If your low back can bend backwards quite well (what we call 'extend'), and if it can bend to either side well (more importantly, if it bends to the sides equally well left and right), then when you sit (and forces such as gravity, muscle contraction and the elastic tension of your ligaments and connective tissues compress your low back into extension) your low back absorbs this energy like a spring.
Now, if everything is happening like the above then, firstly, you don't notice too much discomfort when you are sitting and, secondly, when you come to stand up and move, your back releases its stored energy and you come completely upright without any strain at all.
But if the low back can't extend enough, or has side bending that is different one side to the other, then this spring mechanism does not happen. The back 'warps' and twists as it sits, making it more likely that tissues such as muscles, ligaments and the connective tissue around the joints of the low back will grumble, get increasingly irritated and finally broadcast pain messages.
And, when you come to stand up, because the low back was not compressed into extension in the proper way, there is no effortless recoil - instead, tired achy low back muscles have got confused and, for this reason and others, you have that bent forward look as you come upright and the need to 'walk it off'. Your low back has that heavy feel - whereas the happy low back feels light and floaty.
Note that we haven't talked about the infamous intervertebral disc as a source of sitting problems. All the above can happen in a low back which has perfectly fine discs but just has stiffness and restrictions that prevent it extending and side bending properly.
But it is true that if you do have a problem like this, and you sit a lot, then the abnormal compression of the spine will put a markedly greater strain through the lumbar discs that you would have got from a low back that was sitting more naturally. Often these 'mechanical' causes of disc degeneration speed up the changes already occurring in the discs because of smoking, diabetes, health issues, the ageing process and so on.
Also note, that the discussion above says nothing about the way we sit. See my other blog articles about this important issue.
To conclude - the role of the osteopath is to restore good extension and symmetrical and good ranges of side bending. It is as simple as that. Doing this successfully should mean better sitting, and a happier low back while you are standing walking and running. Even more, your low back will be easier when it is lying down in bed.
You can't 'see' the way your low back bends, so a good tip is to ask the osteopath to video before and after treatment. Except in the most subtle cases, you will notice the difference between the two videos.