Pretty much nothing, at least for steel. You still need the carbon to incorporate into the alloy so like arc furnaces, they can only be used with mostly scrap material. That’s why steel makers still use coal. They need the coke.
That’s the historical reason, but now it's used because carbon is a reducing agent that binds to oxygen, preventing it from oxidizing the iron back into iron oxide and because the coke is used as a permeable membrane to let the resulting gasses escape.
If it was just a matter of heat, there's any number of cleaner fuels steelmakers could use but they can't because the coal serves an important role in the chemistry of blast furnaces.