It's infuriating, isn't it, how difficult it is to discuss this in silence? If we were all in a room together (admittedly, it would have to be a big room), we could say, "Look, if I make a sound like this..." and we'd know what we were talking about. As it is, we're reduced to giving examples of people we've all heard, which gets a bit personal.
All I can usefully add is that it's not just a matter of singing high or low. It's possible to sing the same phrase in the same key in a "head-voice" and then in a "chest-voice" and it'll sound quite different. Chest-voice singing tends to make the pitch sound lower than it is, while head-voice singing makes it sound higher.

Breathiness is a different issue but tends to happen with head-voice singing - I think it would be more difficult to sing breathily with chest-voice, although I dare say it can be done. As for why people do it, I wonder if it's because they got praised for sounding like that when they were children, and just went on singing the same way when they grew up?

Now just let me try that breathy head-voice thing... are you listening?

Marje