Many of the songs from ancient times through the Renaissance were built on the melodies of previous songs. What were the advantages for composers of using a melody that had already been created?

The advantage for composers of using a melody that had already been created is that they already know what it sounded like. It will be easier to incorporate into the songs that the composers are going to make. This will minimize "writer's block". It is a condition where writers are unable to proceed in their work.