On a single- or multi-spool engine, the front fan turning would be an indication that air is moving through the engine in the proper direction before fuel and ignition are introduced. If there was no airflow or it was going the wrong way (such as with a tailwind) light-off would overtemp the engine or possibly blow out the front of the engine. On aircraft with less automated start-up procedures, you have to wait for positive indication of rotation (gauges, visual of the fan), before the fuel valves are opened.