I suspect it was done because people saw other performers do this and assumed that in order to be taken seriously as a 'traditional' singer, and felt they had to follow suit and trust that there was some good reason for it.

A bit like placing a lighted cigarette in the headboard of one's guitar, which seem to make you a better and more earnest blues player.

It would be interesting to establish the history of these practices. Who were the people setting the example?

In truth, I do not seem to see it done much now. If it is done now at all, it tends to be reifly and not done throughout the song. Which lends me to believe that it was always more fashion than passion.

Or possibly a signal, in a noisy place that someone was trying to sing? You could stick the finger in another orifice but it may give an entirely differnt signal?