What is actually meant by balance ? Yes we all know that the gun is "supposed to balance on the joint pin , but why ?

I think you have to consider so many points, length of stock , length of barrel , type of action .
Most modern O/U's with multi chokes seem barrel heavy , so weighting the stock to balance will make the gun very heavy .

Do you personally like the gun to be barrel heavy so that it "swings better " or may be slightly stock heavy so that the gun "prods"? for that quick instinctive point and pull shot .

I have always thought that a gun handles best when the weight seems to be between your hands when mounting , so that the for hand does not have to carry the gun .This point is not always the same as the accepted balance point so many talk of .

Just a personal though , disagree if you like but like so many things shotgun related , that have been accepted as "fact ", having no real basis in anything other than a theoretical ideal, often repeated by authors, so passing into gun lore .