Well, I’ll give you the benefit of the doubt, Opal. Let me amend my rule to state that such a technique is only as useful as its resultant benefit, and in the absence of any perceived benefit it shall be dubbed an affectation.
Professional in the studio: Technique
Caterwauling auditioner: Affectation
I find it’s most useful when singing along with the radio or something where it’s easy to lose track of your own voice in the other sounds. Putting your finger in/over one ear lets you more easily distinguish your own voice from the other noise (you also hear your voice slightly differently and possibly more accurately). I don’t think it would have quite as much effect if you’re singing all by yourself in a quiet room. I’m rather surprised that everyone doesn’t know this trick.
The only time I’ve used it is when I’m working on a harmony, and I need to hear the other voice but not have it overpower my own. It’s silly when you’re doing it on your own though. If you can’t keep in tune when you’re singing by yourself and with no other notes to confuse you, you probably should stop singing. 
Right. It doesn’t do much good unless there is other music/sound. I was simply pointing out that I didn’t think it was an affectation of an invisible “ear monitor” as had been suggested.