By this I don't refer to such obvious devices as a Christmas poem in the shape of a bell, or any poem in any external concrete form. Let's look at the formats that have stood the test of time and are still in use. The sonnet is a solemn form, used to explore emotional themes and conclusions. Heroic couplets in which each two lines rhyme seem usefull to teach or preach. You could never do that with the quick, laughing form of the limerick nor with the dance-like repetition of the villanelle. Can you imagine Robert Services ballads in any other form? The line length and rhythm make an excellent conveyance for his tales. They would not lend themselves to express the solemnity found in a sonnet, nor the wit and dash of a limerick. Each form provides a well understood basis for its expression.What happens too often in so-called free verse is that beneath all the words and images there is no trace of any structure. A poet who begins from structure and language and poetic devices will have so much more at hand to impart his meaning: literal, symbolic, emotional, or hidden. Certainly there is no rule that structure should be evident but a poem that holds together the way such forms tend to hold them will be that much stronger in its impact.
Even though you cannot immediately see the structure underlying a poem, you should be able to feel it. It is the backbone, the strength of a good poem. Structure carries meaning and also makes the poem memorable.