What are some of the various medieval literary forms? why were writers employing these literary forms?

An example can be alliteration or the Slavic antithesis or similar. Very often some of these pieces of writing were poems such as epic poems that would be sung by the people who were most often illiterate. Writers and poets would have to be sung by the entire epics to the listeners which are why these devices are used. They make it easier to remember because they act as formulas that the poet can follow a pattern and by repeating them all over at the same time and changing some events and names he could remember more or if he forgets they would give him time to make something up and make up for the losses.