This guy works for red hat and his code is already constantly merged to upstream. His version is simply more bleeding edge. Also he's already somewhat famous in Linux gaming circles.

It has fixes and features not in the official proton, but most importantly, proprietary codecs.

Proton-GE includes media libraries that valve can't include, which fixes video playback in many older games. Valve has found a different way to bypass this issue, by sharing video content through their shader distribution system, but that may not work for all titles.

Proton-GE also supports more experimental features long before they get added to regular proton, and includes various hotfixes for games that won't work under normal proton.

Usually the recommendation I make to people playing on linux is to try with regular proton or proton experimental first, but if that doesn't work then try the newest proton-GE instead.

I had the same question. Why is this needed? Valve already ships a Proton 11.0 (Beta) version in the Steam client.