Just a hypothesis: some tar evolves at the beginning when coal is pyrolysed, and some tar later, which is less volatile.
Customers may prefer one type. To coat roads for instance, tar with a higher melting point and a low vapour pressure seems better. So the pig iron industry would separate tar evolving early from tar evolving late in order to sell at least one fraction.
Tar is unhealthy as it comprises many aromatics. If it has a higher vapour pressure, the vapour ends in lungs.