What's the breathing sound a boxer makes?

[Hermit]

Only by pride cometh contention: but with the well advised is wisdom

Pride is nothing without honour & honour is respect.

[Shaman Of Sexy]

What is the "shhh" sound a boxer make while striking. I understand that it's him exhaling on a punch, but why does he make it audible? I heard it's for beginners to work on their breathing, and ditch the habit of holding your breath, but then I don't understand why pro boxers still do it.

I think Karate fighters say kiai too, don't they?

I'm just wondering why they make their breathing audible? Does this not use more energy then necessary. They could simply could be exhaling in silence.

I'm curious of the actually benefits of exhaling on a strike. I heard it makes you actually hit harder. Is this a myth or true?

Whenever i train I'd do the sound by myself without knowing.