A web page shouldn't act like it has exclusive ownership of system resources like audio; there may be other pages or applications running, and it's often useful to be able to control them independently.

For instance, if I have Gmail open in the background, there's no reason that adjusting the volume of whatever's in the foreground should also force me to change the volume of incoming chat notifications.

On this note, it's annoying that OS X doesn't provide an external per-app volume control like Windows has. For example, Gmail's voice chat doesn't have a volume control; if I'm watching an iTunes video at the same time (along with the person I'm voice chatting with) and want iTunes to be louder than the chat (due to background noise from the latter), the best I can do is turn the iTunes volume up, but setting it to maximum is insufficient. With such a feature, I could just turn down the browser's volume, though I suppose it's still worthwhile to have volume controls directly embedded in video players for convenience.

You said the same thing that I posted just now, only better. Web pages and applications should never assume that they are being used in isolation and without any switching between them.

I know it sounds weird, but sometimes I have more than one thing on my computer at a time! And some of those things have audio. So it's nice to control the volume of a specific thing, like a video, without having to change the audio for everything else on my system.

It would be nice to turn down the volume on the video I'm playing now, but still have my VoIP app ring loud enough for me to notice it if I'm not paying attention.

I don't know about you, but I would use it for volume control.

I would use the volume control to control the volume.