The ballads collected by Child are presumably the ones that got performed often enough that many people could at least sing along on the chorus; but when and where did these performances occur? Who sang, e.g., "The Elfin Knight", and on what occasions? Were there defined places and times when it was expected that some singing would occur, say Saturday night at the pub, or on market days, or something similar? If so, who decided the program? Would there be instruments accompanying the singing?
If there were professional-ish singers, how did they learn their repertoire? Memorising the words of, say, "The Twa Corbies" is not very difficult if you can read and re-read them on a convenient screen, but seems more challenging if you have to have someone who already knows it repeat the words every time you want to go through it again. So what were the mechanics of oral transmission? (Assuming Child was correct that there was an older oral tradition which had been written down into broadsheets).
(The question also applies to other oral-tradition songs not collected by Child.)
Child was collecting examples of the folk ballad, a specialized form of oral tradition that, like legends and folktales, incorporated a wide range of beliefs. The ballad has roots in medieval Europe, combining narrative and song. Traditional ballads usually focused on a single incident, and they almost always emphasizes action; i.e. something happens in the narrative.
In Britain, there was a late tradition of publishing traditional ballads on "broadsheets," which facilitated the distribution of specific ballads but also regularized the lyrics, which might otherwise vary from one performer to the next.
Ballads were a type of folk performance, in this case undertaken by musical specialists who could carry a tune, remember lyrics, and presumably played an instrument (or knew someone who could!). Most of Britain did not have a strong tradition of professional folk performers, although they were not unknown, particularly in the Celtic fringe. The singing of ballads - whether professionally or not - were a ubiquitous form of entertainment, but obviously, one needed an audience that was gathered, quiet, and willing to listen.