The last thing they want is for some Quest innovation to be hamstrung waiting for a standard body to approve support for some new feature or for a third-party open source maintainer to incorporate some fix/addition/compatibility PR.

Owning a project like this, if they can get developers to use it, is a huge business security asset and competitive advantage for them. That's why this exists and why it's being promoted.

Whether developers see it as worth using instead of other options is a different question. Meta would probably say the answer to "why" for developers is: "to make sure your project gets the most out of the Quest products (as well as other Android platforms), now and in the future"