When a user opens an image they are primarily interested in viewing the image, not in editing tools, file operations or meta data. Hiding the side bar in view mode leads to a nicer default viewing experience.
The editing tools of the side bar are already more easily discoverable than they used to be because of the new button in the default toolbar for showing them.
In browse mode on the other hand everything in the "Information" sidebar tab can also be accessed through the "Properties" action both in the "Operations" sidebar tab or in the right-click context menu. So users should be able to edit the meta data even if they won't see by default that an "Information" sidebar tab even exists. With the "Show Editing Tools" toggle action in the toolbar and the menu bar or hamburger menu they should be able to find whatever else they are looking for.
Having too many panels and bars visible by default doesn't automatically lead to better discoverability of all the functionality because users will start to tunnel vision on the image they are viewing instead of trying to make sense of all the UI elements.
This MR also makes it so that the visibility of the sidebar is conserved between browse mode and view mode. Previously you could have had the sidebar set to visible in one of the modes and not in the other which lead to the sidebar switching visibility "by itself" when switching mode. After this change the sidebar visibility will only change when a user explicitly changes it.
Exempt from this is the fullscreen mode for which the sidebar visibility continues to be saved separately because I believe that in 99% of cases users do not want to use the sidebar when viewing images in fullscreen mode so we do want the sidebar to hide itself by default when switching to fullscreen mode.