Imagery is especially important in poetry because of its compressed form. This means that poems tend to be short (compressed), unlike prose, which is written using sentences. Since poems are so short by nature, poets try to use as few words as possible to describe their thoughts and feelings.
This is where imagery comes into play - imagery refers to the use of very few, and very descriptive words to convey an idea. Instead of using lengthy sentences to describe something, this could be done in poetry using just a few vivid words which clearly paint a picture in our mind. This is done to shorten the length of a poem, while also keeping all relevant information.