Why? Not sure if anyone else encounters this but I have a few times when I pinged someone asking for a review and then they mentioned they are already reviewing it. I thought to myself, "wouldn't it be neat to see who is currently reviewing?". Also as a reviewer, I think it would be nice for the author to know that I'm currently reviewing. Not sure if this resonates with anyone else.
The way it works is, once you install the extension, you'll need to sign in to GitHub. Then, whenever you open a pull request, the extension connects to a live room for that PR over a websocket. You’ll see who else is viewing it, and when someone goes idle (no activity, or they switched tabs) or leaves. And due to the nature of it, you can only see others that also have the extension installed.
I think it's kinda neat (maybe i'm just biased) but feel free to let me know what you think.