Line length shows the reader how it should be read. Short lines are usually read faster, with more emotion. Longer lines slow down the pace of a poem. Choosing appropriate line breaks gives a reader a chance to take a natural breath.
Stanzas, the groups of lines, are like paragraph in prose. They contain a central idea. Having multiple stanzas gives readers a chance to focus on multiple ideas. Think about a page with writing. Is it more manageable to read it if all the words flow together as one paragraph or if they are broken apart into appropriate paragraphs? The same works with poetry.
Structure also refers to the consistency used throughout the poem. An author might start each line with a certain part of speech, or a repeated line or phrase is used at the same spot in each stanza.
When a poem has a strong sense of structure, it flows from beginning to end, and the ideas are easily conveyed.
Examples of poems with a strong structure: