In China there was a story of a Tang Dynasty poet, Jia Dao, who couldn't decide whether to use the word "tap" or "push" in a line of poetry. The story led to the phrase "tap and push" to refer to a careful choice of words in poetry. (Source: A Dream of Red Mansions) A famous American poet (I think it was T.S. Eliot, but I can't find the quote so I don't know for sure) talked about spending all day trying to decide whether or not to keep a comma. The flow of sounds is as important to a poem's memorability as the meaning. Consequently, word choices that would be completely inconsequential in prose will make or break a poem. Add to that that words considered synonyms have different shades of meanings, different connotations, and different allusions that shape the impression and overall meaning of a poem, and it's easy to see why making good poetry is such a challenge.
A basic knowledge of linguistics can be helpful in figuring out why one word works better in a poem than another word. One poem I wrote, for example, had a line that didn't quite sound right. It was "the sunlit frosts which rob the breath". I never have had a very good handle on which contexts take "which" instead of "that," so I wasn't sure which word I should use. But looking at the phonology of the words, I decided that "frosts that rob" sounds better than "frosts which rob," since the the distance in the method and position of articulation between the alveolar affricative "ch" and the retroflexive glide "r" is a more difficult transition than the final "t" in "that" (which becomes a glottal stop in this context, at least in my dialect of English) and the "r". And it could sound like "frost switch"
The latest draft of the poem in question:
Yesterday it snowed all day,
And then the storm was past.
It is the still and cloudless colds that last,
The sunlit frosts that rob the breath,
Taking life without bringing death,
That stay, and stay, and stay.
