They use the moon or sun
As part of their navigation.
One can always fly towards the moon,
But never reach it.
But if one flies towards a white lightbulb,
Keeping it in the corner of the eye,
One keeps flying towards it from any angle.
Yellow light is less attractive to bugs,
Perhaps because it is a component of flames,
Or perhaps because it is often a component of reflected light,
As from grass.