If asked what is necessary in a poem, I always think of imagery. It helps to make a line, a phrase or even a stanza come alive.
For example, you might be speaking about a strong wind that blew trees as well as other things about. If you were simply telling someone about this strong wind you might say ‘the wind last night was noisy, it broke thin boughs and alarmed the cat.’ However, by the use of imagery you bring the sense of all that to life, as in the following few lines:
The wind sang in the trees last night