The concluding lines of ‘Rhonabwy’s Dream’ constitute a colophon (a statement at the end of a book or manuscript giving information about the text, such as its commissioner, its title, or its purpose).
It is generally accepted that all but one of the stories in The Mabinogion belonged to an oral tradition, which saw them passed down through the generations by the storyteller (cyfarwydd). The exception is ‘Rhonabwy’s Dream’ which, it is believed, was created as a literary work in its own right rather than being the transcription of a story transmitted orally.
The colophon implies that the literary form is superior to the oral form. It also suggests that some of the complex descriptive material in the story is there to defeat the oral storyteller, and to show the inadequacies of the oral mode of transmission. However, as anyone who has read 'How Culhwch Won Olwen' (one of the oldest stories in The Mabinogion) will agree, complexity was clearly not something that deterred the cyfarwydd (the traditional Welsh storyteller)!