(just another guest)

Following in from Jim's suggestion, I wonder wonder how many were carried by wandering* traders and tradesmen for whom a good voice and repertoire of songs might engender hospitality, perhaps rather than suspicion as a stranger.

That might encourage conversion of songs into the local language or other forms of acculturation.

Did the Onion Johnnys, for example, sing?

I am avoiding using the word 'traveller', though some may be Travellers in Jim's sense

By the way, regarding the name of the book, only a few paragraphs into the Introduction Roud says "instead of asking 'What folk songs did people sing', we are more concerned with the question 'What songs did the folk sing?'"