Why were writers employing these literary forms?

An example can be alliteration or the slavic antithesis or similar. Very often these pieces of writing were poems such as epic poems that would be sung to the people who were most often illiterate. Writers and poets would have to sing the entire epics to listeners which is why these devices are used. They make it easier to remember because they act as formulas that the poet can follow like a pattern and by repeating them all the 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.