The option under experimental settings quite clearly warns against using it:
Enable the new ‘glimmer’ post stream implementation in ‘auto’ mode for the specified user groups. This implementation is under active development, and is not intended for production use. Do not develop themes/plugins against it until the implementation is finalized and announced.
So the wording is pretty strong in saying that it “is not intended for production use. Do not develop themes/plugins against it”
That’s the explanation for the option Glimmer post stream mode auto groups.