A metaphor makes a comparison, and in doing so shapes our perception. If we say, "Time is a river," we're noting a certain similarity between the two. Yet we know they aren't identical. We may mean that time is fluid, has currents and eddies, empties into some vast ocean, but not that it's composed of water. If we say, "Time is a stone," we may mean it's silent, still, indifferent, but not that it's a mineral.
Unlike prose, which wraps freely from paragraph to paragraph, poetry is written in lines, which break in definite spots selected by the poet. Because of this, the line is considered a unit of composition, an important factor influencing sound and meaning.