It’s a long and fascinating story, the development of this line of thinking. The formulaic nature of so many Homeric epithets and images has inclined scholars to the thought (inspired by a study of extant forms of oral poetry in the Balkans) that the bards (known as rhapsodes) used these prefabricated tropes as modules with which they could build the poem’s structure as they recited. Think of jazz. The rhapsodes knew the melody and the key, as it were, and could therefore vamp at will.
