which is the structure and literary form being used and how and why this form works to promote the underlying purpose of the text ?

The Song of Roland consists of roughly 4000 lines of verse, divided into 298 poetic units called laisses. Laisses are irregular in length, from three or four lines to a few hundred, but in The Song of Roland they average under fourteen lines. The lines are mostly decasyllabic, and are connected by assonance (the last word contains a similar vowel sound but not necessarily a perfect rhyme) or by rhyme.

One of the poem's striking features is the use of parallel laisses, in which consecutive laisses echo slightly different versions of the same event. The term is not precise; its key characteristic is a slowing of the pace of narrative and a formula of repetition. We can see this technique at work in the scene where Oliver climbs the hill.

Another striking feature of the poem is its paratactic structure. Rather than connect sentences with conjunctions, the poet lays down lines one after the other with no connecting words. This kind of format is known as parataxis. Causality and connection between phrases are almost always implicit; this form runs throughout the poem.

Please check this out in the GradeSaver link below,