Two perfectly machined flat surfaces will sit flat to each other and not leak. Add a gasket, especially with hot exhaust and expansion / contraction with heat cycles and you have a big potential for a gasket blow out and leak. Once there is a leak the hot exhaust gasses will actually erode the machined surfaces. To the best of my knowledge muscle era cars used no exhaust manifold gaskets. Gaskets on exhaust manifold a came into vogue more when aluminum heads became popular, and are included in gasket kits for older engine as a bandaid for warped and damaged surfaces.
