Heating the mould reduces chilling of the material as it's introduced, thus allowing it to flow & fully fill the mould - matching the surface finish of the mould.
A similar surface effect can be seen when casting lead bullets, the surface finish fails to match that of the mould until the mould is brought up to working temperature - with bullet casting this is done either by repeatedly casting & discarding product until the results come good or by pre-heating the mould.