A long time ago, I read somewhere, that there is a very specific reason, why we put salt in the water for cooking pasta: The point is to hinder the water from absorbing flavor and nutrients from the pasta. With soup its the other way around: We want to absorb the flavors into the water, which is why we salt it only at the end.
The explanation was, that salt "ionizes" water, which somehow makes it less likely to absorb things.
Is there any truth to this theory?
Because pure water draws salts and other soluble nutrients from the interior of vegetables, salting vegetable cooking water also minimizes nutrient loss.