The idea would be to make coaling a bigger part of the game, at least as an option for some people to specialize
Increasing the quality of steel would be very valuable wouldn't it?
I guess the problem is you don't need that much coal for steel but just a small amount of high quality coal, so no one
would actively run 4 to 30 tar kilns just to sell all the coal.
If tar kilns get more functionality i think it'd be cool to have this, but I can see why it's not appealing to everyone.