The book of Lamentations is a story of what happened and what one man felt when the darkness fell upon the Jewish people in 586 BC.
This final chapter, this final lament is more chaotic than all the others. While it has 22 verses, one for each letter of the Hebrew alphabet, unlike the other 4 chapters there is no acrostic, no order to the verses. Some speculate that this is a picture of the chaos the author felt. In the end he just pours out his pain in the rawest of forms.
At some level this is what grief is. It is pouring out our pain through our words and expressions. We can either bottle up the poison, or find a way to express our sorrow. These lamentations give us permission to pour out our sorrow.