In poetry, dissonance, is a powerful and useful literary device, employed by poets, although dissonance disrupts the harmony of vowel sounds in a text. It is the opposite of assonance. Dissonance occurs when the poet uses vowel sounds to cut the rhythmic flow of sounds, causing an uneasy or disturbing feeling. The sounds could often be harsh to the listener or reader. This device is, especially, of use to a poet when the poet wishes to cause uneasiness to the reader or listener in order to convey something important that is very relevant, in the poem's context. Here's an example of dissonance from Ted Hughe's poem Wind. The dissonance calls attention to the havoc causing aspect of the wind.
“At noon I scaled along the house-side as far as
The coal-house door. Once I looked up –
Through the brunt wind that dented the balls of my eyes
The tent of the hills drummed and strained its guyrope…
The wind flung a magpie away and a black-
Back gull bent like an iron bar slowly.”
(Wind by Ted Hughes)