The poem is all about the qualities of a best friend. A best friend is like an angel sent by God to be with you all the times of your life. A best friend is a true friend who stay with you through all your ups and downs of your life. Such friend is indeed a best friend.
A best friend is one of a kind person in your life. He is with you in all your needs and problems. He also understands you best. You cannot live without your best friend beside you.
Alliteration-
the occurrence of the same letter or sound at the beginning of adjacent or closely connected words.
Consonance-agreement or compatibility between opinions or actions.
"consonance between conservation measures and existing agricultural practice"
synonyms: agreement, accord, harmony, unison; More
the recurrence of similar sounds, especially consonants, in close proximity (chiefly as used in prosody).
MUSIC
the combination of notes that are in harmony with each other due to the relationship between their frequencies.
plural noun: consonances
Assonance-
in poetry, the repetition of the sound of a vowel or diphthong in nonrhyming stressed syllables near enough to each other for the echo to be discernible (e.g., penitence, reticence ).
The contribution of these sound devices to the poem are to give rhyme and sounds between words. These also gives emphasis to the meaning of the poem about.