It was a fashionable technique among ‘60s folkies, variously to help hear themselves sing/find the right note against other noises around (like someone singing off-key nearby) or to look pretentious, depending on your point of view.
Some believed it added authenticity to a performer, much as how sticking a lit cigarette in the headstock of one’s guitar was supposed to add intensity and earnestness and imposed gravitas and credibility.
E is also for eclecticism
The mixing/fusing/blending of styles new and old has been a feature of artistic ventures forever, but musically it took off in the 1960s when Dylan, The Byrds, Joni Mitchell and other musicians began the experimenting.
Mixing traditional melodies, verse forms, ballads and narratives from folk music with rock, coupled with social, political and politico-social themes became not only the norm, but hugely commercially successful, and laid down paths that most others have followed since. Any definition of folk music today can only be described by including the concept of eclecticism.
John Martyn (1948-2009) was described by the UK’s Times as: “an electrifying guitarist and singer whose music blurred the boundaries between folk, jazz, rock and blues”.
To give just one contemporary example, Paul Russell admits on his website that he plays “eclectic acoustic folk music infused with elements of ragtime, country, jazz, tomfoolery, classical, celtic, delta blues and pirates.” Russell's influences include Martin Sexton, The Decemberists, Michael Hedges, Iron and Wine, Leonard Cohen, Nickel Creek, Darrell Scott, Keller Williams, Tom Robbins and Rob Brezsny.
Listen (and watch): Leonard Cohen inspiring eclecticism in others - The Stranger Song (live in 1967)