Covered (or Plateau) keys are easy to use. The pad cup is covered so it will seal the tonehole any time the key is closed. This is common with beginners or others who may not always press the center of the key.
Many advanced players prefer the open feel of Ring keys for subtle control of their tone. Intermediate and Standard model French system flutes come with key plugs to seal the hole until players have become adept at covering the holes with their fingers.