Files: Unify the Bookmarks and XDG directories in the sidebar and allow removal
The XDG directories are always present in the sidebar and not removable whether or not they are relevant to someone's use case which means the side bar has several items populating it regardless, taking up a lot of vertical real estate. Additionally, if you bookmark some locations they automatically start pretty far down the sidebar (due in part to the XDG list) and any mounted directories/devices will additionally push your bookmarks further down the sidebar.
If we unify the list of bookmarks and XDG directories, folders relevant to people remain at the top of the sidebar and if we allow removing of XDG directories that aren't used (including the behaviour to let them be re-added at any time as you would a normal bookmark) the sidebar locations remain more relevant at all times.
Relevant Art
Finder on Mac OS treats all directories as "favourites" including any standard directory:
On Windows, File Explorer's "Quick Access" can pin any directory as well, but it is worth noting that the standard places are their own thing on Windows as "Libraries".
/cc @aday