This feature can be particularly useful for stopping all automatic playback of annoying videos or for controlling a YouTube video if you switch to another tab. This would also be a welcome change, given that Google recently removed the ability to disable tabs by clicking on the speaker icon, even if you can still right-click on a tab to disable the sound permanently. Global multimedia controls are more versatile, especially for enraged observers on YouTube, and you will not need to look for offensive auto-play tabs.
After playing with the feature for a short while, I discovered that she could control both audio and video, but not on all sites (it did not work with Engadget, for example). In addition to that, the functionality was extremely buggy, planting almost constantly when I was trying to use it.
However, the first tests are still in progress and if Google decides to include it in the main browser, it will definitely work. If you want to try it yourself, you can download the Canary browser here and then activate the flag chrome: // flags / # global-media-controls