On the topic of how this helps out Godot, one of our goals with The Mirror is to be an open platform. We want to use open standards whenever possible so that content made on or for The Mirror can be used elsewhere. One big example of this is we want to put more data inside of GLTF files, such as physics data, and have it be interoperable between games and game engines. We have contributed this enhancement to Godot https://github.com/godotengine/godot/pull/69266 and there are many other engine contributions The Mirror has made to Godot (mostly from myself).
On the topic of what features this offers. The Mirror has real-time collaboration for developing content, which most platforms do not offer. For example, there is no way to have a "multiplayer" Godot editor, with multiple people editing a scene at the same time and seeing live changes. You can think of it like Garry's Mod, but built on Godot and using open standards.
Another example of open standards, The Mirror has Ready Player Me integration, which is a website that lets you design a character and they give you a URL to download a .glb file. You can paste this URL into The Mirror and then you're playing as that avatar. The same avatar can be brought into other Godot games if they add support for it.