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.
A poem's form is partly visual: its look on the page. George Herbert's "Easter Wings" is an example of striking visual form, as is e. e. cummings' "r-p-o-p-h-e-s-s-a-g-r." But visual form also works in less obvious ways. The lean, spare look of most Emily Dickinson poems complements her terse style, while the long, sweeping lines of Walt Whitman accentuate his bold, expansive message.