A threefold purpose might be considered:
1. It functions as a memory aid, because each poetic verse begins with a new letter of the Hebrew alphabet.
2. A complete alphabet conveys the fullness and depth of lament, “from A to Z.”
3. The use of acrostics enables the poet to devote his full attention to the theme at hand.1
1 How lonely sits the city that was full of people! How like a widow has she become, she who was great among the nations! She who was a princess among the provinces has become a slave.