What purpose did the use of alliteration and rhyme serve in the ballads from the anglo-saxon era?

In prosody, alliterative verse is a form of verse that uses alliteration as the principal ornamental device to help indicate the underlying metrical structure, as opposed to other devices such as rhyme. The most commonly studied traditions of alliterative verse are those found in the oldest literature of the Germanic languages, where scholars use the term 'alliterative poetry' rather broadly to indicate a tradition which not only shares alliteration as its primary ornament but also certain metrical characteristics. The Old English epic Beowulf, as well as most other Old English poetry, the Old High German Muspilli, the Old Saxon Heliand, the Old Norse Poetic Edda, and many Middle English poems such as Piers Plowman, Sir Gawain and the Green Knight, and the Alliterative Morte Arthur all use alliterative verse

Ibelieve it is entry b.