Id like to try and get rid of this idea that MacColl put his finger in his ear. He didnt. He cupped his hand behind his ear...........lots of singers do it....Mike Waterson being another well known example. Take a look at the frontpage cover of "Journeyman". no finger in ear.
Some people find it helps them hear themselves - especially when singing harmonies. A " finger in the ear" is about as useful as an ear plug when singing (IMHO)