Every poet uses specific literary devices for their own personal reasons. Instead of trying to figure out why a specific poet used a specific device, an easier way to answer your question is to identify, define, and explain the use of common literary devices used in poetry.
Alliteration is the repetition of a vowel sound within a line of poetry. Poets typically use alliteration to give a poem a sing-song feeling. Alliteration was commonly used in the epic poetry of the Medieval period given writing ability was not something common for all. Instead, the poems of this period were performed by scops, singers who would perform for the courts and tell tales of the epic hero.
Metaphors and similes are comparisons between two (or more) unlike things. Metaphors do not use the words "like/as" in the comparison, whereas similes do. The use of metaphors and similes typically allow a poet to create a visual or sensual image for a reader. The poet wants a reader to understand their texts. Creating a comparison normally helps a reader to engage or understand texts better. Other times, the use of these devices is needed to create an imbalance between things to give the reader another way to look at an abstract or concrete object or idea.
Repetition is the repeated use of a word, phrase, or sound. Typically repetition is used so that a reader does not fail to see the importance of the idea or statement being made. Repeating a word, or phrase, embeds it in the mind of the reader. Basically, the author is insuring that the reader does not miss exactly what the author is wanting to point out.
Imagery is perhaps the most importnat weapon in the artillary of a poet. Imagery attacks the any, or all, of the five senses of the reader. This attack on the senses provides a picture for the reader. If a poet is depicting a dark and fearful place the words an author could use would be shadowed, empty, desolate, foreboding, or evil.
While there are many, many more poetic/literary devices, the use of each depends solely upon what the poet wishes to convey in their writing. No poet is the same- much like their works. Therefore, the use of the devices can never be for the same exact reasons.