Good poems ask us to have complex minds and hearts. Even simple-of-surface poems want that. Perhaps those are the ones that want it most of all, since that's where they do their work: in the unspoken complexities, understood off the page.
Metaphors think with the imagination and the senses. The hot chili peppers in them explode in the mouth and the mind.
The pressed oil of words can blaze up into music, into image, into the heart and mind's knowledge. The lit and shadowed places within us can be warmed.
Metaphors get under your skin by ghosting right past the logical mind.