Еврейская мысль к Берешит 41:41

Now that vegetation had been achieved without sunshine or moonlight, the lightbodies could begin to perform their orbits in order to provide warmth for living creatures that were to inhabit the earth and its atmosphere. Winds too are a prerequisite for the normal existence of the animal kingdom. The directives for the lightbodies to be "in the sky," clearly demonstrates that they had already been in existence, but were now to assume the functions described, namely to provide light, directions, indicate seasons etc. This is the "why" of their coming into existence. For this reason the Torah could not say " va-yehi me-orot” and there came into being lightbodies." This directive then determines the "when" which is part of the ten definitions listed at the outset the significance of the ten directives.