I think it would be nice if the sidebar could be entirely collapsed, like it can on many other apps. This gets it out of the way, leaving more room for the content area, and can be especially useful when you have multiple windows on-screen at once.
Specifically in my case I have someone using Zoom and having a meeting agenda in Fibery in view at the same time. They want to have Fibery on one side of their screen, so they can see the agenda and take notes, and they want Zoom on the other side so they can see people they’re talking to. Currently the sidebar is taking up a lot of width unnecessarily in this case, so the Zoom window is smaller than would be ideal.
Also it’s often quicker to navigate using Search, which can make the sidebar unnecessary a lot of the time. So that’s another advantage.