Imagery is a vivid and descriptive language that helps us evoke pictures or ideas in our minds that appeals to either of our five senses: sight, hearing, smell, taste or touch. An example of a sentence using this language is "Romina, tired as she was, sat down on her bed and simply hugged the soft and warm pillow." Here, the adjectives "soft" and "warm" used for the pillow create a mental picture in our minds and appeal to our sense of touch.
So, a poet most likely uses imagery in a poem to add beauty, to help the readers create pictures or ideas in our minds that appeals to our senses, and helps us feel more connected to the literature work.