I have done this when singing harmonies, as it helps me hear my own voice "inside my head," esp. on the first few run-throughs of a new piece in a choir-type situation. I can't really see the point of doing it solo; that would honestly seem a mere affectation. [I could draw a parallel with the other finger, but this is a family site.] Mildly tangentially, I have never been able to wean myself away from keeping my ear to the frame when I play autoharp. Holding an ear to the top of a guitar body is comforting too, in a foetal kind of way I guess. Helps check the tuning if there's a lot of background noise.

CC