Not only the poets, but also the common people speak in a figurative way.
When we say someone is a pig, we do not literally mean that person is a
pig; just like when we use the expression "raining cats and dogs," we do
not mean cats and dogs do fall from the sky like raindrops. The figurative
language is "not merely a literary mannerism; it is based on the way our
body and mind put us in touch with the universe" (Nims 22).
  It is not necessary for poetry to employ figurative language, but if it
does (in other words, showing the reader more concrete images), it often
could improve the readability of itself; and the reader could understand
it more easily. Or the words would become pretty dull, illustrating nothing
but the plain truth, like what Professor Raleigh comments on Wordsworth's
poetry: "But the mere fact, which says everything, comes perilously near
also to saying nothing" (Alden 137).