"The Rime of the Ancient Mariner" is full of symbols. Some of these, like the sun, change their meaning, as the mariner and his shipmates change their minds about the wisdom of killing albatrosses. The sun goes from being a benign provider of light and warmth, and a useful measure of time, to being an instrument of torture for the mariner as it bakes him alive:
All in a hot and copper sky,
The bloody Sun, at noon,
Right up above the mast did stand,
No bigger than the Moon.
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Concerning Coleridge's "The Rime of the Ancient Mariner," one can't really speak for a writer and say why he or she does something. All we can do is talk about the effects of what a writer does.
"Rime" is a work of imagination, and it has as its theme a love of and reverence for nature. Indeed, the Mariner sums up the theme by telling the wedding guest that one must respect all of God's creatures.
One shouldn't be surprised, then, at the prevalence of nature imagery. What starts out as natural in the poem, becomes the supernatural during the course of the poem. Any repetition you notice creates unity and serves other purposes, as well. For instance, the Mariner comes to love the sea snakes, revealing his transformation.
Death is the penalty for disrespecting God's creatures. And again, this is depicted via Coleridge's imagination.
Imagery is used to reveal plot, theme, tone, mood, etc., and it is repeated to create unity, and for additional purposes as well, depending on the specific images.