Coleridge said that poetry is the best words in the best order. I see a "boiling down" process in literature. The novel has the largest scope, and the individual words are not always that important (although you could make a case that each one counts since it a choice). Drama has fewer words; poetry (lyric, at least) the fewest. Words are our interface with "reality"; they are not reality, but they are the only way we can express anything about it. One of the things that poetry attempts to do is mediate that realtionship; in the process it can help us "see" things that were always there, but never quite obvious. A simple example: In Dickenson's poem "The Bustle in a house," the use of the word bustle is wonderful. It describes the hectic preparation for a funeral, the kind of frenetic activity that attemps to take our mind off the horrible reality of the night before, but which, like most bustle, never really achieves its hoped for end. The image of "sweeping up the heart" takes what has become a somewhat trite expression, a broken heart" and makes us see that, although it is broken, it can be saved and we can use again in "eternity." These specially chosen words in an 8 line poem contain incredible simply simply because they carry the weight of meaning in a way that is almost "perfect." Dickinson has a lot of poems that function this way, but I'm out of space :)
The word choice is important in writing poetry because:
1. Poetry is a concentration of ideas. Proper word choice is necessary for presenting the exact idea.
2. In some cases word choice is necessary for maintaining rhyme.
3. Symbolic significance of a particular aspect is presented through proper word choice.
The above are obviously my opinion and I believe this topic needs to be posted as a forum post for a varied response.