These days I tend to sing unaccompanied into mics at places I'm pretty sure people are going to shut up and listen most of the time, and hear myself pretty well. But I resorted quite naturally to finger in ear recently while at band practice, when I couldn't hear myself for the others playing, until I realised I could turn myself up!

To all those who think it's an affectation - nope, it really does work, and in noisy places can be crucial, I imagine.