There’s often not much benefit to more horizontal content on the screen, at one point it starts to make reading less efficient… Readability: The Optimal Line Length – Articles – Baymard Institute
At the moment I would say you gain focus when you hide the sidebar — the sidebar provides some light notifications when there is new content in various places, and this can be distracting for some people.
It’s also completely expected that the sidebar can be a “set it and forget it” feature — someone can toggle it on or off and never really think about it again. Though always interested to hear more perspectives on this.