Why are Flight Directors turned on so early? I have noticed that the requirement to turn on the Flight Directors in a commercial aircraft is in one of the very first checklists. Why does this have to be the case when the Flight Directors are really just an indication of where the aircraft needs to fly to?

Why, for instance, is it not in the "Before Take-off" checklist?
For a jetliner, the 'why early' is to test the auto flight system. Turning on the F/D also turns on the Flight Mode Annunciator (FMA) and displays the armed modes.

So it's an early check that all systems are go.

From a 777 manual, any fault will be indicated on the FMA as it reads above.

Here are the flow checks pertaining to the F/D and FMA during the cockpit setup: