Answer:
They have an idea that they'd like repeated throughout the poem.
Step-by-step explanation:
A villanelle is a highly structured poem consisting of nineteen lines with a specific rhyming pattern and repeated lines. The repetition of lines and the specific structure of the poem allow the poet to emphasize a specific idea or theme throughout the poem. This makes the villanelle an ideal choice for poets who want to reinforce an idea or concept through repetition.
In contrast, a sonnet is a fourteen-line poem with a specific rhyme scheme and structure. While a sonnet can also be used to reinforce a particular idea or theme, its structure and rhyme scheme may not be as conducive to repetition as a villanelle.
Therefore, a poet may choose a villanelle instead of a sonnet if they have an idea or theme that they would like to repeat throughout the poem.