Using "skies" instead of "sky"

Skies is a poetic/literary word used to mean heaven or heavenly power. The first example sentence could mean reach for heaven.

In some set phrases, the used word is skies, as in He wrote to his sister praising Lizzie to the skies. In this case, to the skies means very highly or enthusiastically.

According to the OED, the earliest meaning of "sky" in English was "cloud" (which is what "sky" still means in Scandinavian languages).

From that "the skies", originally meaning "the clouds", came to mean "heaven", or "sky" (in the modern sense).

The OED lists "sky" in the modern sense as the third meaning. It doesn't spell it out, but I think that what happened is that once "the skies" took on the meaning of "the expanse of heaven", it was no longer felt to be plural, so people started referring to it as "the sky". If this is correct, then "the skies" is simply an older form, which survives in poetic use and a few phrases.