The 'spare pages' collection
The 'spare pages' collection is... a collection of spare pages. These are already created, and published - but not visible in the menus - and they could be simply deleted, if they are unused.
But for now, they exist. Here they are, created and editable, as an insurance policy, in the (unlikely?) event that after trying a few pages using the 'Gutttenberg' editor, we find that we cannot get back to the classic editor, which is used for all the pages until now.
If we can't get back, but do want to add a few more pages in this 'classic editor' styling, then we can re-edit these and put them into the menus, so that they will appear to a viewer.