The feature could be particularly handy for stopping any annoying auto-play videos or controlling a YouTube video if you switch to another tab. It would also be a welcome change, considering Google recently removed the ability to mute tabs by clicking on the speaker icon, though you can still right-click on a tab to permanently mute a site. Global Media Controls are more versatile, though, particularly for rabid YouTube watchers, and you won't need to chase down any offending auto-play tabs.
After playing with the feature for a short time, I found that it could control both audio and video, but not on all sites (it didn't work with Engadget, for instance). On top of that, the feature was extremely buggy, crashing pretty much constantly when I attempted to use it.
However, it's still in early testing, and if Google decides to bake it into the main browser, it will no doubt work seamlessly. If you want to give it a shot for yourself, you can download the Canary browser here, then enable the flag chrome://flags/#global-media-controls