Koji mold (fungus) is necessary for the brewing of sake. Koji mold is a change agent, ie Koji will break down the starch molecules (long chains) into small chains of molecules and eventually into sugar. The sake yeast will then take over and convert those rice sugars into alcohol and CO2.
Typically, only a tiny portion of the steamed rice is inoculated with Koji mold. The Koji mold will then start its "magic". This sake is fully inoculated with Koji, leading to a quicker breakdown of starch and making more sugar molecules available for the "yeast attack."
This method gives the sake a much more intense flavor and aroma and a balance of sweetness, tartness and acidity.