Yax said:
I believe it's because you can use the lungs more efficiently if you bend the back.
No, you can't. Time for a basic human anatomy lesson.
The lungs operate by motion of the diaphragm, a large muscle which separates the lung cavity from the lower internal organs. When this muscle pushes
down, the lungs are drawn down with it. This decreases the air pressure in the lungs, as any air left there expands with the lungs. The lower air pressure causes air from outside the body to be drawn in, to equalize the pressure.
Now here's the really important part:
In order for the diaphragm to push down, it has to push down into your guts. This compresses the lower organs, and causes the belly to expand outward. Take a look for yourself. Relax completely and breathe naturally; you will see your stomach going in and out as you breathe.
Some people seem to think you breathe by expanding your chest. This is not true. You have no muscles which can contract to make your chest expand. Also, you have a rib cage which won't expand very much at all. (It does expand some small amount if you completely fill your lungs, as the cartilage which connects bones is flexible - but the bones themselves can't bend.) When you draw a deep breath, it may feel like your chest is expanding a great deal, but it isn't. Watch yourself breathe in a mirror for proof.
Now on to the bending...
When you bend forward, you make it more difficult to take a deep breath because the belly can't expand as easily. Try it. Take a deep breath standing up, then another one bent over. You'll see that you breathe much more effeciently in an upright posture. I've seen Halford live, and he really doesn't bend forward enough to make a huge difference - but there is no way in hell he's breathing better in that posture than if he were standing straight up.
So why does Halford do it, aside from the teleprompter?
Answer: He's not just bending his back forward. He's also bending his head down towards his chest. This shortens the length of the throat, a necessary step for singing very high notes.
When singers are young, they can use their throat muscles to move the larynx up, shortening the length of vibrating air to hit the high notes. But as a singer, I can tell you that doing so becomes much more difficult with age. When I do it, I can hit the high notes for a while ... but my throat tires quickly. Listen to me sing
"Stagefright". You can hear me doing OK at the start, but by the second chorus I'm just not able to really hit those high notes anymore. Or ask Ian Gillan, who won't sing "Child In Time" anymore because his throat muscles can't do it. (And no one could ever accuse Gillan of having a weak throat! It's just age.)
So Halford bends his neck to make his throat's job easier, allowing him to sing those high notes much longer than if he stood up straight all the time. His bending gives his throat a break.
) looks like he's in pain, & the whole band look like they're fed up with what they're doing & can't be bothered anymore, as if it's something they had to do. I can't explain it, but it seems like there is something terribly wrong with it. 
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