The purpose of imagery in poetry is to help get the poet's message across in language that is strong, vivid and very visual. The poet will use words to create images in our heads that help us to interpret the poem in the way he sees it. Every person has a different view on life and poets are inspired to record theirs for others to read and identify with. Sometimes, such as in "Vampire" by Ted Hughes, the images will be gruesome - as in a party host with  a slack mouth like a gaping sack. Sometimes, such as in Shakespeare (Sonnet 18) the images are delicate and beautiful as in "shall I compare thee to a summer's day." Sometimes poets use similes in comparison, sometimes they use personification and sometimes metaphor. All create visuals for us, whether to shock or delight.

There will be many different approaches to answering this question.  I would say that one of the primary purposes of imagery in poetry is to help create mental pictures through words that help illuminate the poet's effectiveness.  Imagery helps to create a series of visualizations of one's mind while they are reading.  This helps to capture the essence of meaning in writing and catapult the relevancy of the author.  For example, in Langston Hughes' poem "Harlem," he uses imagery or mental pictures to help bring out the answers to his question, "What happens to a dream deferred?"  Images like the raisin drying up in the sun or crusting over "a syrupy sweet" or festering "like a sore," are all examples of pictures created through words that enhance meaning in the poem.

There are numerous answers to your question about the purpose of imagery in poetry.  I'll mention one that is particularly relevant to poetry, as opposed to prose.

There probably is no accurate definition (in our current literary climate) of poetry, that covers everything that today is considered poetry.  For instance, what we accept today as prose poetry doesn't fit any standard definitions from the past of what poetry is.  If there is a single characteristic that at least fits most of what is considered poetry, it may be compressed language.

Poetry has a need to say as much as possible in as few words as possible.  That is true with most successful writing, of course, but the need is more pronounced in poetry.

Imagery is an important form of compressed language, and is therefore vital in poetry.  Other forms of compressed language, such as simile and metaphor, often create imagery.  They help a writer to accomplish as much as possible in as few lines as possible. 

Consider Randall Jarrell's "The Death of the Ball Turret Gunner":

From my mother's sleep I fell into the State

And I hunched in its belly till my wet fur froze.

Six miles from earth, loosed from its dream of life,

I woke to black flak and the nightmare fighters.

When I died they washed me out of the turret with a hose.

Looking at only the final line, the image of fellow soldiers washing the leftover bits of a man ripped to shreds by shrapnel creates multiple meanings, reactions, and emotions:

• Horror
• Shock
• Disgust
• Disbelief or at least discomfort at the nonchalance that seems to be involved
• anti-war sentiment
• irony
• pity

Notice that none of the words I use to describe the meanings, etc. of the image are used by the writer:  they are all revealed by the image.  That's what imagery does for poetry. 

Imagery is figurative language that embellishes poetry; specifically, it is the representation through language of sense experience.  By appealing to the senses, the poet is able to evoke the emotions that accompany these sensations.  This appeal is more indirect that the appeal to the senses of music and rhythms; nevertheless, imagery delights the following senses:

• visual
• auditory
• olfactory
• gustatory (taste)
• tactile
• organic  (internal- hunger, thirst, fatigue, nausea)
• kinesthetic (movement or tension in the muscles or joints)

Imagery is at the core of how people perceive the world; thus, this figurative language influences the reader of a poem to the meaning that the poet desires through its appeals to the senses of the reader which enable him/her to share the poet's vision.