Why did Anne Bradstreet consider poetic devices essential when writing her poetry?

We have a lot of records on some poets. Some poets spell out their goals for their poetry in detail, explaining what they are doing and why. In other cases, we have more humble records, such as accounts of the difficulty a poet had capturing an image, or the experience that sparked a poem. With Bradstreet, we don't have this sort of record. She didn't theorize poetry or keep accounts of her struggles with composition. We must therefore work from two lines of argument to answer your question.

The first line of argument is general: how period poets wrote. Colonial poets like Bradstreet were part of a larger Anglo-American context. Colonial poets followed English models of the preceding century. These poets used poetic devices extensively, as did the other colonial poets, including Bradstreet. With this line of argument, you'd conclude by saying Bradstreet felt it was essential to use poetic devices in her poetry because that is what a poet writing in her time did. That's what poetry was. The best-known poets of that time (and the preceding decades) all used rhyme, regular rhythm, metaphor, and allusion.

A second line of argument comes from the evidence in Bradstreet's own work, like "The Author to Her Book." In that poem, she describes the process of writing poetry. Lines like "I stretched thy joints to make thee even feet" work both as metaphors and direct comments on poetry. Those "feet" are the feet a poet might walk on, but also the rhythm of poetry. In this line of reasoning, Bradstreet used poetic devices as the best way to communicate her meaning and create art.