Why does Christian Bale change his voice when playing Batman

Well, most people often forget how easy it is to recognize somebody by voice and Bruce Wayne is kind of a celebrity. So changing his voice is a natural consequence of Nolan's making Batman more realistic. Of course Superman can't be Clark Kent, as he's obviously missing the characteristic glasses ;)

And well, it surely also contributes to his dramatic appearance, but this more as a second goal, I think.

I do agree that disguise is one reason. But this is Nolan, there's always more. In Batman Begins, Bruce tells Alfred:

Bruce Wayne: People need dramatic examples to shake them out of apathy and I can't do that as Bruce Wayne, as a man I'm flesh and blood I can be ignored I can be destroyed but as a symbol, as a symbol I can be incorruptible, I can be everlasting.

Alfred Pennyworth: What symbol?

Bruce Wayne: Something elemental, something terrifying.

Though, the consequent scenes do not talk much about the symbol part and everybody pretty much forgets Bruce's original idea of Batman. We start thinking of Batman as Bruce with a costume on and fighting the bad guys. Which is true but to Bruce it's something more, like he says to Alfred. It's a symbol to terrify people.

If you were a villain it wouldn't be easy for you to think that Batman is just a guy with a mask on. That makes it more terrifying. The idea of someone powerful than the ordinary human. And Bruce takes every possibility to make the idea a very real and unshakable threat to the bad guys.

The voice, the bat ears, the dramatic entrances and exits - all these are trivial to the plot. Either way it's not going to change the output of a scene if Batman enters sneakily with a very chilly line or just through the door. But Bruce takes the extra step to go through all the mumbo-jumbo just to keep the idea real. The idea of a legend. The idea of a permanent warrior who is going to fight the bad guys every turn and forever.

It all makes sense. Come to think of it. Joker says something similar to:

"You'll be outcast too. They'll see you as a freak when they are done."

Joker is a schizophrenic villain who wears face paint all day long and he empathizes with Bruce. He actually identifies himself with Batman. You can understand the similarities - Bruce goes to great extents to protect the man behind the mask. Same thing goes for Joker - he wears face paint 24x7. Without it, he wouldn't be as scary. And scary is important to him.

Summary: Bruce takes the idea of Batman more seriously than the audience are led to believe. The outcome of it is one thing - crimefighting. He does take that seriously. But he also takes the very idea of Batman personally and he is proud of it.

Probably a bit of both. The main reason would be to disguise his voice so that people don't recognise him as Bruce Wayne. But in selecting the how to disguise his voice he would want it to sound intimidating. It would make for a very different movie if he sounded all cute and fluffy!

Well, firstly, it must be noted that this was actually not Christian Bale's overacting, but Nolan's directing. He even went as far as to push bale to growl more while filming and even more in post. There was a cut detail about the cowl housing some apparatus that disguises his voice to what it sounds like. Being that this is a Nolan move, I'm sure there is a symbolic purpose behind this voice. I would venture to say the batman voice is as different to Wayne's as the characters themselves. All Batman mythos harps on the duality of he character, or characters, rather.

MTV was able to ask Bale about that process of his bat-voice while he is out doing press for his new movie Out of the Furnace.

"I got there. They put me in Val Kilmer's suit. It didn't even fit properly, and I stood in it and I went 'I feel like an idiot.' What kind of guy walks around, dressed like a bat? And is then going to go 'Hello, how are you? Just ignore that I'm dressed as a bat.' Of course, he's meant to be doing this," Bale said. "If you look at the history of the guy and the pain that he went through. I went 'I can't do this in a normal voice. I have to become a beast in order to sell this to myself.'"

you can see his first Batman Audition in the link down below:
Bale's Batman Audition

What I got from it was his voice gets lower and darker as the films go on. This to me sounds like he is becoming what he is fighting against. Which was a major theme in dark knight and dark knight rises.

Then again he could just be doing it for reasons stated above. But Nolan seems to have a reason for everything in a film. My guess would be he was becoming or he would have to become a bad guy in order to take down other bad guys. In the dark knight he said as much.

I can usually identify actors I have seen before by their voice, while getting things out of the fridge in the next room. I would hate to be a millionaire philanthropist who was on TV or even radio (People still listen in their cars ya'know !) and have someone recognise my voice after I had gone through so much effort to hide my face. (I would NOT mind being a millionaire if no one knew me though!) I recognized Bane in something else just a few days ago by voice alone.

Bruce REALLY should have a "wonderful toy" to do this for him by now. With all the money he spends on high-tensile strength cable, a voice changer is cheap by comparison!