You drain the fuel is to check if the fuel has impurities and water.
Why should you do it first thing in the morning, before fly or before refuelling?
That is becuase during the night all the impurities sinks to the bottom and the water too ( water has heigher density than fuel). So if you move the plane or you refuel th plane all the impurities and water will be suspended again. If you refuel or move the aircraft you donnt have to drain because the result may be worng. As you know the drain is at the lowest part of the fuel tank.
