Not saying you won't be able to invoke the volume control widget on a screen, but when harnesses are always listening, models are lower latency and you're interacting with a device from the other side of the room in a lot of cases (even PCs, via 72" wall mounted monitors), it'll be a lot more ergonomic.

There was a study that showed users "felt" faster on a keyboard even while completing tasks faster on a mouse. Similarly, with low latency models, "chromium volume 20%, actually, 10%" will be faster than clicking the sound control icon, finding the chromium volume from the list and reducing it, not to mention you don't have to look away from what you're doing. Direct interaction will probably only be faster if you need extremely precise control, for example, you wouldn't do digital painting with an agent, which seems pretty intuitive.