Functionality, all this appears to do is hide the timeline and notification bar, since the sidebar is already collapsible. I don’t really see the experience being changed or improved sufficiently to have another button in every topic.
It would make more sense if it changed the reading experience in some meaningful way like making it full width in Reader Mode. As it is now, I would probably never use this feature so ironically all it’s doing for me is adding more visual noise to the timeline.